test(send_message): add thread-not-found retry tests for Telegram topics

Three tests covering the #27012 fix:
- test_is_thread_not_found_matches_expected_errors
- test_text_send_retries_without_thread_id_on_thread_not_found
- test_disable_web_page_preview_not_leaked_to_media_sends

116/116 existing tests still pass (no regressions).

tests/tools/test_send_message_tool.py

@@ -23,6 +23,7 @@ def _reset_signal_scheduler():
 from gateway.config import Platform
 from tools.send_message_tool import (
     _derive_forum_thread_name,
+    _is_telegram_thread_not_found,
     _parse_target_ref,
     _send_discord,
     _send_matrix_via_adapter,
@@ -2478,3 +2479,94 @@ class TestCheckSendMessage:
              patch("gateway.status.is_gateway_running",
                    side_effect=ImportError("simulated")):
             assert _check_send_message() is False
+
+
+class TestSendTelegramThreadNotFoundRetry:
+    """Tests for thread-not-found retry behaviour in _send_telegram (#27012)."""
+
+    def test_is_thread_not_found_matches_expected_errors(self):
+        """_is_telegram_thread_not_found should detect thread-not-found errors."""
+        class FakeError(Exception):
+            pass
+
+        assert _is_telegram_thread_not_found(FakeError("message thread not found")) is True
+        assert _is_telegram_thread_not_found(FakeError("THREAD NOT FOUND")) is True
+        assert _is_telegram_thread_not_found(FakeError("Bad Request: thread not found")) is True
+        assert _is_telegram_thread_not_found(FakeError("chat not found")) is False
+        assert _is_telegram_thread_not_found(FakeError("parse error")) is False
+        assert _is_telegram_thread_not_found(FakeError("")) is False
+
+    def test_text_send_retries_without_thread_id_on_thread_not_found(self):
+        """When thread is not found, the text send should retry without
+        message_thread_id."""
+        call_args = []
+
+        async def fake_retry(bot, *, chat_id, text, parse_mode, **kwargs):
+            call_args.append(dict(kwargs, chat_id=chat_id, text=text))
+            if len(call_args) == 1:
+                raise Exception("Bad Request: message thread not found")
+            return SimpleNamespace(message_id=42)
+
+        async def run_test():
+            with patch(
+                "tools.send_message_tool._send_telegram_message_with_retry",
+                fake_retry,
+            ):
+                # _send_telegram imports Bot locally; we only need to mock
+                # the send path, not Bot itself (Bot import falls through
+                # normally since python-telegram-bot is installed).
+                return await _send_telegram(
+                    "fake-token", "-100123", "hello from topic 17585",
+                    thread_id="17585",
+                )
+
+        result = asyncio.run(run_test())
+        assert result["success"] is True
+        assert result["message_id"] == "42"
+        assert len(call_args) == 2, f"expected 2 calls, got {len(call_args)}"
+        # First call should have message_thread_id
+        assert call_args[0].get("message_thread_id") is not None
+        # Second call (retry) should NOT have message_thread_id
+        assert "message_thread_id" not in call_args[1], \
+            "retry should drop message_thread_id after thread-not-found"
+
+    def test_disable_web_page_preview_not_leaked_to_media_sends(self):
+        """disable_web_page_preview should only appear in text send, not media sends."""
+        text_kwargs_seen = []
+        media_kwargs_seen = []
+
+        class FakeBot:
+            async def send_message(self, **kwargs):
+                text_kwargs_seen.append(kwargs)
+                return SimpleNamespace(message_id=1)
+
+            async def send_document(self, **kwargs):
+                media_kwargs_seen.append(kwargs)
+                return SimpleNamespace(message_id=2)
+
+        import tempfile
+        media_path = None
+        try:
+            with tempfile.NamedTemporaryFile(suffix=".pdf", delete=False) as tf:
+                tf.write(b"%PDF-1.4 test content")
+                media_path = tf.name
+
+            async def run_test():
+                with patch("telegram.Bot", return_value=FakeBot()):
+                    return await _send_telegram(
+                        "fake-token", "-100123", "check preview",
+                        media_files=[(media_path, False)],
+                        disable_link_previews=True,
+                    )
+
+            result = asyncio.run(run_test())
+            assert result["success"] is True
+            # Text send should have disable_web_page_preview
+            assert text_kwargs_seen[0].get("disable_web_page_preview") is True
+            # Media send should NOT have disable_web_page_preview
+            assert "disable_web_page_preview" not in media_kwargs_seen[0], \
+                "disable_web_page_preview leaked into send_document kwargs"
+        finally:
+            if media_path and os.path.exists(media_path):
+                os.unlink(media_path)
+