fix(bigquery-magics): wait for graph server startup before returning from init

[parthea]

GraphServer.init() starts the server on a background thread and returns immediately. If stop_server() is called right after init() (for example, in fast unit tests that don't make HTTP requests), self._server may still be None. When that happens, stop_server() does nothing and server_thread.join() hangs indefinitely in serve_forever().

This PR adds a threading.Event so init() waits until self._server is assigned before returning.

[gemini-code-assist]

Code Review

This pull request introduces a synchronization mechanism using threading.Event to ensure that GraphServer.init() blocks until the background TCP server is fully initialized, resolving a race condition with stop_server(). A unit test was also added to verify this behavior. The feedback suggests avoiding an indefinite wait on self._server_ready.wait() by implementing a polling mechanism with a timeout, which prevents the main thread from hanging if the background server thread fails to start (e.g., due to port binding errors).