fix: add warning log when session ID is unknown or expired

Closes #2204

The else branch in _handle_stateful_request() silently returned 404 when
receiving an unknown or expired session ID. The other branches already
log at debug/info level. This adds a warning log to help operators
diagnose stale session ID issues (e.g. after server restarts).

Author: Alberto Farah

src/mcp/server/streamable_http_manager.py

@@ -270,6 +270,10 @@ async def run_server(*, task_status: TaskStatus[None] = anyio.TASK_STATUS_IGNORE
                 await http_transport.handle_request(scope, receive, send)
         else:
             # Unknown or expired session ID - return 404 per MCP spec
+            logger.warning(
+                "Rejected request with unknown or expired session ID: %s",
+                session_id,
+            )
             # TODO: Align error code once spec clarifies
             # See: https://github.com/modelcontextprotocol/python-sdk/issues/1821
             error_response = JSONRPCError(