fix(core): make OGXAsLibraryClient thread-safe

[zy1o]

What does this PR do?

Rework the sync-on-async client.

The previous implementation called loop.run_until_complete() on a shared loop instance inside .request() method. When multiple threads called request() concurrently, this raised RuntimeError: This event loop is already running.

The fix uses a single dedicated daemon thread running loop.run_forever(), with all coroutines submitted with a asyncio.run_coroutine_threadsafe() call.

Known limitations and tradeoffs

• Not fully thread-safe: per @mattf's comment on OGXAsLibraryClient is not thread-safe #5752, global state in the async client may still cause races under concurrent load. This PR fixes the event loop collision specifically.
• Uses a bounded threading.Queue with busy-poll fallback (asyncio.sleep(0.01)) when full. My judgement call is: This is good enough for LLM token rates.
• future.cancel() on the streaming coroutine is best-effort; a coroutine blocked on network I/O won't respond immediately.
• Decided against the use of janus to avoid extra dependencies.

Closes #5752

Test Plan

Added unit tests covering:

• Concurrent requests from multiple threads.
• Init failure cleanup (background thread stops on failed initialization).

Launch just them with:

pytest tests/unit/ -vs -k TestOGXAsLibraryClientSyncOnAsync