fix(gateway): avoid cross-user mirror writes in per-user group sessions

tests/tools/test_send_message_tool.py

@@ -167,6 +167,39 @@ class TestSendMessageTool:
             media_files=[],
         )
 
+    def test_mirror_receives_current_session_user_id(self):
+        config, _telegram_cfg = _make_config()
+
+        with patch("gateway.config.load_gateway_config", return_value=config), \
+             patch("tools.interrupt.is_interrupted", return_value=False), \
+             patch("model_tools._run_async", side_effect=_run_async_immediately), \
+             patch("tools.send_message_tool._send_to_platform", new=AsyncMock(return_value={"success": True})), \
+             patch("gateway.session_context.get_session_env") as get_session_env_mock, \
+             patch("gateway.mirror.mirror_to_session", return_value=True) as mirror_mock:
+            get_session_env_mock.side_effect = lambda name, default="": {
+                "HERMES_SESSION_PLATFORM": "telegram",
+                "HERMES_SESSION_USER_ID": "user-123",
+            }.get(name, default)
+            result = json.loads(
+                send_message_tool(
+                    {
+                        "action": "send",
+                        "target": "telegram:12345",
+                        "message": "hello",
+                    }
+                )
+            )
+
+        assert result["success"] is True
+        mirror_mock.assert_called_once_with(
+            "telegram",
+            "12345",
+            "hello",
+            source_label="telegram",
+            thread_id=None,
+            user_id="user-123",
+        )
+
     def test_top_level_send_failure_redacts_query_token(self):
         config, _telegram_cfg = _make_config()
         leaked = "very-secret-query-token-123456"