feat(gateway): Telegram reply threading modes (off/first/all)

Add configurable reply_to_mode for Telegram multi-chunk replies:
- off: never thread replies to original message
- first: only first chunk threads (default, preserves current behavior)
- all: all chunks thread to original message

Configurable via reply_to_mode in platform config or TELEGRAM_REPLY_TO_MODE
env var.

Cherry-picked from PR #855 by raulvidis, rebased onto current main.
Dropped asyncio_mode=auto pyproject.toml change, added @pytest.mark.asyncio
decorators, fixed test IDs to use numeric strings.

Co-authored-by: Raul <77628552+raulvidis@users.noreply.github.com>

tests/gateway/test_telegram_reply_mode.py

@@ -0,0 +1,210 @@
+"""Tests for Telegram reply_to_mode functionality.
+
+Covers the threading behavior control for multi-chunk replies:
+- "off": Never thread replies to original message
+- "first": Only first chunk threads (default)
+- "all": All chunks thread to original message
+"""
+import sys
+from unittest.mock import MagicMock, AsyncMock, patch
+
+import pytest
+
+from gateway.config import PlatformConfig
+
+
+def _ensure_telegram_mock():
+    """Mock the telegram package if it's not installed."""
+    if "telegram" in sys.modules and hasattr(sys.modules["telegram"], "__file__"):
+        return
+    mod = MagicMock()
+    mod.ext.ContextTypes.DEFAULT_TYPE = type(None)
+    mod.constants.ParseMode.MARKDOWN_V2 = "MarkdownV2"
+    mod.constants.ChatType.GROUP = "group"
+    mod.constants.ChatType.SUPERGROUP = "supergroup"
+    mod.constants.ChatType.CHANNEL = "channel"
+    mod.constants.ChatType.PRIVATE = "private"
+    for name in ("telegram", "telegram.ext", "telegram.constants"):
+        sys.modules.setdefault(name, mod)
+
+
+_ensure_telegram_mock()
+
+from gateway.platforms.telegram import TelegramAdapter  # noqa: E402
+
+
+@pytest.fixture()
+def adapter_factory():
+    """Factory to create TelegramAdapter with custom reply_to_mode."""
+    def create(reply_to_mode: str = "first"):
+        config = PlatformConfig(enabled=True, token="test-token", reply_to_mode=reply_to_mode)
+        return TelegramAdapter(config)
+    return create
+
+
+class TestReplyToModeConfig:
+    """Tests for reply_to_mode configuration loading."""
+
+    def test_default_mode_is_first(self, adapter_factory):
+        adapter = adapter_factory()
+        assert adapter._reply_to_mode == "first"
+
+    def test_off_mode(self, adapter_factory):
+        adapter = adapter_factory(reply_to_mode="off")
+        assert adapter._reply_to_mode == "off"
+
+    def test_first_mode(self, adapter_factory):
+        adapter = adapter_factory(reply_to_mode="first")
+        assert adapter._reply_to_mode == "first"
+
+    def test_all_mode(self, adapter_factory):
+        adapter = adapter_factory(reply_to_mode="all")
+        assert adapter._reply_to_mode == "all"
+
+    def test_invalid_mode_stored_as_is(self, adapter_factory):
+        """Invalid modes are stored but _should_thread_reply handles them."""
+        adapter = adapter_factory(reply_to_mode="invalid")
+        assert adapter._reply_to_mode == "invalid"
+
+    def test_none_mode_defaults_to_first(self):
+        config = PlatformConfig(enabled=True, token="test-token")
+        adapter = TelegramAdapter(config)
+        assert adapter._reply_to_mode == "first"
+
+    def test_empty_string_mode_defaults_to_first(self):
+        config = PlatformConfig(enabled=True, token="test-token", reply_to_mode="")
+        adapter = TelegramAdapter(config)
+        assert adapter._reply_to_mode == "first"
+
+
+class TestShouldThreadReply:
+    """Tests for _should_thread_reply method."""
+
+    def test_no_reply_to_returns_false(self, adapter_factory):
+        adapter = adapter_factory(reply_to_mode="first")
+        assert adapter._should_thread_reply("12345", None, 0) is False
+        assert adapter._should_thread_reply("12345", "", 0) is False
+
+    def test_off_mode_never_threads(self, adapter_factory):
+        adapter = adapter_factory(reply_to_mode="off")
+        assert adapter._should_thread_reply("12345", "123", 0) is False
+        assert adapter._should_thread_reply("12345", "123", 1) is False
+        assert adapter._should_thread_reply("12345", "123", 5) is False
+
+    def test_first_mode_only_first_chunk(self, adapter_factory):
+        adapter = adapter_factory(reply_to_mode="first")
+        assert adapter._should_thread_reply("12345", "123", 0) is True
+        assert adapter._should_thread_reply("12345", "123", 1) is False
+        assert adapter._should_thread_reply("12345", "123", 2) is False
+        assert adapter._should_thread_reply("12345", "123", 10) is False
+
+    def test_all_mode_all_chunks(self, adapter_factory):
+        adapter = adapter_factory(reply_to_mode="all")
+        assert adapter._should_thread_reply("12345", "123", 0) is True
+        assert adapter._should_thread_reply("12345", "123", 1) is True
+        assert adapter._should_thread_reply("12345", "123", 2) is True
+        assert adapter._should_thread_reply("12345", "123", 10) is True
+
+    def test_invalid_mode_falls_back_to_first(self, adapter_factory):
+        """Invalid mode behaves like 'first' - only first chunk threads."""
+        adapter = adapter_factory(reply_to_mode="invalid")
+        assert adapter._should_thread_reply("12345", "123", 0) is True
+        assert adapter._should_thread_reply("12345", "123", 1) is False
+
+
+class TestSendWithReplyToMode:
+    """Tests for send() method respecting reply_to_mode."""
+
+    @pytest.mark.asyncio
+    async def test_off_mode_no_reply_threading(self, adapter_factory):
+        adapter = adapter_factory(reply_to_mode="off")
+        adapter._bot = MagicMock()
+        adapter._bot.send_message = AsyncMock(return_value=MagicMock(message_id=1))
+        
+        # Mock truncate to return multiple chunks
+        adapter.truncate_message = lambda content, max_len: ["chunk1", "chunk2", "chunk3"]
+        
+        await adapter.send("12345", "test content", reply_to="999")
+        
+        # Verify none of the calls had reply_to_message_id
+        for call in adapter._bot.send_message.call_args_list:
+            assert call.kwargs.get("reply_to_message_id") is None
+
+    @pytest.mark.asyncio
+    async def test_first_mode_only_first_chunk_threads(self, adapter_factory):
+        adapter = adapter_factory(reply_to_mode="first")
+        adapter._bot = MagicMock()
+        adapter._bot.send_message = AsyncMock(return_value=MagicMock(message_id=1))
+        adapter.truncate_message = lambda content, max_len: ["chunk1", "chunk2", "chunk3"]
+        
+        await adapter.send("12345", "test content", reply_to="999")
+        
+        calls = adapter._bot.send_message.call_args_list
+        assert len(calls) == 3
+        
+        # First chunk should have reply_to_message_id
+        assert calls[0].kwargs.get("reply_to_message_id") == 999
+        # Remaining chunks should not
+        assert calls[1].kwargs.get("reply_to_message_id") is None
+        assert calls[2].kwargs.get("reply_to_message_id") is None
+
+    @pytest.mark.asyncio
+    async def test_all_mode_all_chunks_thread(self, adapter_factory):
+        adapter = adapter_factory(reply_to_mode="all")
+        adapter._bot = MagicMock()
+        adapter._bot.send_message = AsyncMock(return_value=MagicMock(message_id=1))
+        adapter.truncate_message = lambda content, max_len: ["chunk1", "chunk2", "chunk3"]
+        
+        await adapter.send("12345", "test content", reply_to="999")
+        
+        calls = adapter._bot.send_message.call_args_list
+        assert len(calls) == 3
+        
+        # All chunks should have reply_to_message_id
+        for call in calls:
+            assert call.kwargs.get("reply_to_message_id") == 999
+
+    @pytest.mark.asyncio
+    async def test_no_reply_to_param_no_threading(self, adapter_factory):
+        adapter = adapter_factory(reply_to_mode="all")
+        adapter._bot = MagicMock()
+        adapter._bot.send_message = AsyncMock(return_value=MagicMock(message_id=1))
+        adapter.truncate_message = lambda content, max_len: ["chunk1", "chunk2"]
+        
+        await adapter.send("12345", "test content", reply_to=None)
+        
+        calls = adapter._bot.send_message.call_args_list
+        for call in calls:
+            assert call.kwargs.get("reply_to_message_id") is None
+
+    @pytest.mark.asyncio
+    async def test_single_chunk_respects_mode(self, adapter_factory):
+        adapter = adapter_factory(reply_to_mode="first")
+        adapter._bot = MagicMock()
+        adapter._bot.send_message = AsyncMock(return_value=MagicMock(message_id=1))
+        adapter.truncate_message = lambda content, max_len: ["single chunk"]
+        
+        await adapter.send("12345", "test", reply_to="999")
+        
+        calls = adapter._bot.send_message.call_args_list
+        assert len(calls) == 1
+        assert calls[0].kwargs.get("reply_to_message_id") == 999
+
+
+class TestConfigSerialization:
+    """Tests for reply_to_mode serialization."""
+
+    def test_to_dict_includes_reply_to_mode(self):
+        config = PlatformConfig(enabled=True, token="test", reply_to_mode="all")
+        result = config.to_dict()
+        assert result["reply_to_mode"] == "all"
+
+    def test_from_dict_loads_reply_to_mode(self):
+        data = {"enabled": True, "token": "test", "reply_to_mode": "off"}
+        config = PlatformConfig.from_dict(data)
+        assert config.reply_to_mode == "off"
+
+    def test_from_dict_defaults_to_first(self):
+        data = {"enabled": True, "token": "test"}
+        config = PlatformConfig.from_dict(data)
+        assert config.reply_to_mode == "first"