Fix deadlock in atexit cleanup handlers

[rob0tics]

Fix deadlock in atexit cleanup handlers

Fixes #3004

Problem

When using asyncio_atexit or similar libraries to register async cleanup handlers, Playwright hangs indefinitely during shutdown. The program never exits cleanly.

Root Cause

When atexit handlers are registered, Python's atexit mechanism creates a new execution context at program termination. The _stopped_future in the Transport class is tied to the original event loop, but when atexit handlers run, they attempt to use this future in a different (or already-closed) event loop context. This causes a deadlock - the code tries to await a future from a dead loop that cannot be resolved.

Solution

The fix involves detecting when the event loop is closed or unavailable and handling shutdown gracefully:

1. wait_until_stopped() method: Check if the event loop is closed before awaiting the stopped future. If closed, return immediately since the OS will clean up the process.

2. run() task: Catch asyncio.CancelledError when the event loop shuts down, allowing the message-reading loop to exit gracefully instead of crashing.

3. Shutdown logic: Before attempting any async operations, verify that a running event loop exists. If not, send SIGTERM to the driver process and trust the OS to clean up child processes.

Instead of adding defensive timeouts or force-killing processes, the fix follows the Unix philosophy: trust the OS to clean up child processes. A graceful SIGTERM signal is sent, and the kernel handles process reaping.

[rob0tics]

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