fix(gateway): restore Telegram DM topic thread_id after session split (#27166)

When context compression triggers a mid-turn session split, source.thread_id
can be None on synthetic/recovered events. _thread_metadata_for_source then
returns None, causing the Telegram adapter to send with no message_thread_id
and the response lands in the General thread instead of the active DM topic.

Fix:
- hermes_state.py: Add get_telegram_topic_binding_by_session() for reverse
  lookup by session_id (enabled by the existing UNIQUE INDEX on session_id).
- gateway/run.py: After session-split detection, if source is a Telegram DM
  and source.thread_id is None, recover it from the binding via the new
  method so _thread_metadata_for_source produces the correct thread routing.
- tests/: Coverage for the new lookup method and the recovery flow.

tests/gateway/test_telegram_topic_mode.py

@@ -1229,6 +1229,100 @@ def test_list_telegram_topic_bindings_for_chat_no_table(tmp_path):
     assert tables == set()
 
 
+# ---------------------------------------------------------------------------
+# Tests for get_telegram_topic_binding_by_session (issue #27166)
+# ---------------------------------------------------------------------------
+
+def test_get_telegram_topic_binding_by_session_returns_binding(tmp_path):
+    """Reverse lookup by session_id returns the binding row."""
+    db = SessionDB(db_path=tmp_path / "state.db")
+    db.enable_telegram_topic_mode(chat_id="208214988", user_id="208214988")
+    db.create_session(session_id="sess-27166", source="telegram", user_id="208214988")
+    db.bind_telegram_topic(
+        chat_id="208214988",
+        thread_id="17585",
+        user_id="208214988",
+        session_key="agent:main:telegram:dm:208214988:17585",
+        session_id="sess-27166",
+    )
+
+    binding = db.get_telegram_topic_binding_by_session(session_id="sess-27166")
+
+    assert binding is not None
+    assert binding["chat_id"] == "208214988"
+    assert binding["thread_id"] == "17585"
+    assert binding["session_id"] == "sess-27166"
 
 
+def test_get_telegram_topic_binding_by_session_returns_none_for_unknown(tmp_path):
+    """Returns None when no binding exists for the given session_id."""
+    db = SessionDB(db_path=tmp_path / "state.db")
+    db.apply_telegram_topic_migration()
 
+    result = db.get_telegram_topic_binding_by_session(session_id="nonexistent-sess")
+
+    assert result is None
+
+
+# ---------------------------------------------------------------------------
+# Test for session-split thread_id recovery (issue #27166)
+# ---------------------------------------------------------------------------
+
+def test_session_split_restores_source_thread_id_from_binding(tmp_path):
+    """After a session split, source.thread_id is restored from the binding.
+
+    Simulates the case where context compression creates a new session_id and
+    source.thread_id is None (synthetic/recovered event). The recovery block
+    must look up the binding by the new session_id and restore thread_id on
+    source so that _thread_metadata_for_source returns the correct thread.
+    """
+    from gateway.run import GatewayRunner
+    from gateway.config import Platform
+
+    db = SessionDB(db_path=tmp_path / "state.db")
+    db.enable_telegram_topic_mode(chat_id="208214988", user_id="208214988")
+    db.create_session(session_id="sess-split-new", source="telegram", user_id="208214988")
+    db.bind_telegram_topic(
+        chat_id="208214988",
+        thread_id="17585",
+        user_id="208214988",
+        session_key="agent:main:telegram:dm:208214988:17585",
+        session_id="sess-split-new",
+    )
+
+    runner = object.__new__(GatewayRunner)
+    runner._session_db = db
+
+    # Build a source that looks like it came from a synthetic/recovered event:
+    # platform and chat_type match a Telegram DM, but thread_id is None.
+    source = _make_source(thread_id=None)
+    assert source.platform == Platform.TELEGRAM
+    assert source.chat_type == "dm"
+    assert source.thread_id is None
+
+    # Simulate the session-split recovery block logic directly.
+    if (
+        getattr(source, "platform", None) == Platform.TELEGRAM
+        and getattr(source, "chat_type", None) == "dm"
+        and getattr(source, "thread_id", None) is None
+        and runner._session_db is not None
+    ):
+        try:
+            _binding = runner._session_db.get_telegram_topic_binding_by_session(
+                session_id="sess-split-new",
+            )
+            if _binding and _binding.get("thread_id"):
+                source.thread_id = str(_binding["thread_id"])
+        except Exception:
+            pass
+
+    assert source.thread_id == "17585", (
+        "thread_id must be restored from the binding after session split"
+    )
+
+    # Confirm _thread_metadata_for_source now returns non-None.
+    runner.config = _make_runner(session_db=db).config
+    runner.adapters = _make_runner(session_db=db).adapters
+    meta = GatewayRunner._thread_metadata_for_source(runner, source)
+    assert meta is not None
+    assert meta["thread_id"] == "17585"