fix(gateway): merge rapid TEXT follow-ups during active sessions (#4469) (#26822)

When the agent is running and the user sends multiple TEXT messages in
rapid succession, base.py's active-session branch stored the pending
event as a single-slot replacement:

    self._pending_messages[session_key] = event

Three rapid messages A, B, C landed as: A (interrupts), B (replaces A
before consumer reads), C (replaces B). Only C reached the next turn —
A and B were silently dropped. This is the symptom in #4469.

Route the follow-up through merge_pending_message_event(..., merge_text=True)
so TEXT events accumulate into the existing pending event's text instead
of clobbering it. Photo and media bursts already merged through the same
helper; this just extends the merge_text path (already used by the
Telegram bursty-grace branch in gateway/run.py) to all platforms.

Test exercises BasePlatformAdapter.handle_message directly with the
session marked active and asserts three rapid TEXT events merge to
'part two\\npart three' rather than dropping the middle message.
Sanity-checked the test would fail without the fix.

Credits @devorun for the original investigation and analysis in #4491
that surfaced the underlying queue handling, though their fix targeted
GatewayRunner._pending_messages which is now dead state on main.

tests/gateway/test_active_session_text_merge.py

@@ -0,0 +1,152 @@
+"""Regression test for #4469.
+
+When the agent is actively running (session present in
+``adapter._active_sessions``) and the user fires off multiple TEXT
+follow-ups in rapid succession, the previous behaviour was a single-slot
+replacement at ``gateway/platforms/base.py``:
+
+    self._pending_messages[session_key] = event
+
+So three rapid messages ``A``, ``B``, ``C`` arriving while the agent was
+still working on the initial turn produced a pending slot containing only
+``C``; ``A`` and ``B`` were silently dropped.
+
+The fix routes the follow-up through ``merge_pending_message_event(...,
+merge_text=True)`` so TEXT events accumulate into the existing pending
+event's text instead of clobbering it.  Photo / media bursts continue to
+merge through the same helper (they always did).
+"""
+
+from __future__ import annotations
+
+import asyncio
+import sys
+import types
+from unittest.mock import AsyncMock, MagicMock
+
+import pytest
+
+# Minimal telegram stub so importing gateway.platforms.base does not pull
+# in the real python-telegram-bot dependency.
+_tg = sys.modules.get("telegram") or types.ModuleType("telegram")
+_tg.constants = sys.modules.get("telegram.constants") or types.ModuleType("telegram.constants")
+_ct = MagicMock()
+_ct.PRIVATE = "private"
+_ct.GROUP = "group"
+_ct.SUPERGROUP = "supergroup"
+_tg.constants.ChatType = _ct
+sys.modules.setdefault("telegram", _tg)
+sys.modules.setdefault("telegram.constants", _tg.constants)
+sys.modules.setdefault("telegram.ext", types.ModuleType("telegram.ext"))
+
+from gateway.config import Platform, PlatformConfig
+from gateway.platforms.base import (
+    BasePlatformAdapter,
+    MessageEvent,
+    MessageType,
+)
+from gateway.session import SessionSource, build_session_key
+
+
+def _make_event(text: str, chat_id: str = "12345") -> MessageEvent:
+    source = SessionSource(
+        platform=Platform.TELEGRAM,
+        chat_id=chat_id,
+        chat_type="dm",
+        user_id="u1",
+    )
+    return MessageEvent(
+        text=text,
+        message_type=MessageType.TEXT,
+        source=source,
+        message_id=f"msg-{text[:8]}",
+    )
+
+
+def _make_adapter() -> BasePlatformAdapter:
+    """Build a BasePlatformAdapter without running its heavy __init__.
+
+    We only need the bits ``handle_message`` touches on the active-session
+    path: ``_active_sessions``, ``_pending_messages``,
+    ``_message_handler``, ``_busy_session_handler``, ``config``, ``platform``.
+    """
+
+    class _DummyAdapter(BasePlatformAdapter):  # type: ignore[misc]
+        async def connect(self):
+            pass
+
+        async def disconnect(self):
+            pass
+
+        async def get_chat_info(self, chat_id):
+            return None
+
+        async def send(self, *args, **kwargs):
+            return MagicMock(success=True, message_id="x", retryable=False)
+
+    adapter = object.__new__(_DummyAdapter)
+    adapter.config = PlatformConfig(enabled=True, token="***")
+    adapter.platform = Platform.TELEGRAM
+    adapter._message_handler = AsyncMock(return_value=None)
+    adapter._busy_session_handler = None
+    adapter._active_sessions = {}
+    adapter._pending_messages = {}
+    adapter._session_tasks = {}
+    adapter._background_tasks = set()
+    adapter._post_delivery_callbacks = {}
+    adapter._expected_cancelled_tasks = set()
+    adapter._fatal_error_code = None
+    adapter._fatal_error_message = None
+    adapter._fatal_error_retryable = True
+    adapter._fatal_error_handler = None
+    adapter._running = True
+    adapter._auto_tts_default = False
+    adapter._auto_tts_enabled_chats = set()
+    adapter._auto_tts_disabled_chats = set()
+    adapter._typing_paused = set()
+    return adapter
+
+
+@pytest.mark.asyncio
+async def test_rapid_text_followups_accumulate_instead_of_replacing():
+    """Three rapid TEXT follow-ups during an active session must all
+    survive in ``adapter._pending_messages[session_key].text``."""
+    adapter = _make_adapter()
+    first = _make_event("part one")
+    session_key = build_session_key(first.source)
+
+    # Mark the session as active so subsequent messages take the
+    # "already running" branch in handle_message.
+    adapter._active_sessions[session_key] = asyncio.Event()
+
+    second = _make_event("part two")
+    third = _make_event("part three")
+
+    await adapter.handle_message(second)
+    await adapter.handle_message(third)
+
+    # Both rapid follow-ups must be preserved, not just the last one.
+    pending = adapter._pending_messages[session_key]
+    assert pending.text == "part two\npart three", (
+        f"expected accumulated text, got {pending.text!r}"
+    )
+    # Interrupt event must be signalled exactly like before.
+    assert adapter._active_sessions[session_key].is_set()
+
+
+@pytest.mark.asyncio
+async def test_single_followup_is_stored_as_is():
+    """One TEXT follow-up still lands as the event object itself
+    (no spurious wrapping / mutation) — guards against the merge path
+    breaking the simple case."""
+    adapter = _make_adapter()
+    first = _make_event("only one")
+    session_key = build_session_key(first.source)
+
+    adapter._active_sessions[session_key] = asyncio.Event()
+    await adapter.handle_message(first)
+
+    pending = adapter._pending_messages[session_key]
+    assert pending is first
+    assert pending.text == "only one"
+    assert adapter._active_sessions[session_key].is_set()