fix(gateway): preserve notify context in executor threads

Gateway executor work now inherits the active session contextvars via
copy_context() so background process watchers retain the correct
platform/chat/user/session metadata for routing completion events back
to the originating chat.

Cherry-picked from #10647 by @helix4u with:
- Use asyncio.get_running_loop() instead of deprecated get_event_loop()
- Strip trailing whitespace
- Add *args forwarding test
- Add exception propagation test

tests/gateway/test_session_env.py

@@ -251,3 +251,72 @@ def test_session_key_no_race_condition_with_contextvars(monkeypatch):
     assert results["session-B"] == "session-B", (
         f"Session B got '{results['session-B']}' instead of 'session-B' — race condition!"
     )
+
+
+@pytest.mark.asyncio
+async def test_run_in_executor_with_context_preserves_session_env(monkeypatch):
+    """Gateway executor work should inherit session contextvars for tool routing."""
+    runner = object.__new__(GatewayRunner)
+    monkeypatch.delenv("HERMES_SESSION_PLATFORM", raising=False)
+    monkeypatch.delenv("HERMES_SESSION_CHAT_ID", raising=False)
+    monkeypatch.delenv("HERMES_SESSION_THREAD_ID", raising=False)
+    monkeypatch.delenv("HERMES_SESSION_USER_ID", raising=False)
+
+    source = SessionSource(
+        platform=Platform.TELEGRAM,
+        chat_id="2144471399",
+        chat_type="dm",
+        user_id="123456",
+        user_name="alice",
+        thread_id=None,
+    )
+    context = SessionContext(
+        source=source,
+        connected_platforms=[],
+        home_channels={},
+        session_key="agent:main:telegram:dm:2144471399",
+    )
+
+    tokens = runner._set_session_env(context)
+    try:
+        result = await runner._run_in_executor_with_context(
+            lambda: {
+                "platform": get_session_env("HERMES_SESSION_PLATFORM"),
+                "chat_id": get_session_env("HERMES_SESSION_CHAT_ID"),
+                "user_id": get_session_env("HERMES_SESSION_USER_ID"),
+                "session_key": get_session_env("HERMES_SESSION_KEY"),
+            }
+        )
+    finally:
+        runner._clear_session_env(tokens)
+
+    assert result == {
+        "platform": "telegram",
+        "chat_id": "2144471399",
+        "user_id": "123456",
+        "session_key": "agent:main:telegram:dm:2144471399",
+    }
+
+
+@pytest.mark.asyncio
+async def test_run_in_executor_with_context_forwards_args():
+    """_run_in_executor_with_context should forward *args to the callable."""
+    runner = object.__new__(GatewayRunner)
+
+    def add(a, b):
+        return a + b
+
+    result = await runner._run_in_executor_with_context(add, 3, 7)
+    assert result == 10
+
+
+@pytest.mark.asyncio
+async def test_run_in_executor_with_context_propagates_exceptions():
+    """Exceptions inside the executor should propagate to the caller."""
+    runner = object.__new__(GatewayRunner)
+
+    def blow_up():
+        raise ValueError("boom")
+
+    with pytest.raises(ValueError, match="boom"):
+        await runner._run_in_executor_with_context(blow_up)