v1.16.11-rc7

Fix event loop closed error and add comprehensive UDF dispatch tests

Replace asyncio.run() with _run_with_graceful_shutdown() that drains
pending callbacks before closing the loop, preventing RuntimeError from
httpx/anyio TLS cleanup in async UDFs calling OpenAI/LangChain APIs.

Add 17 unit tests covering graceful shutdown, cancellation timing,
exception propagation, context variable isolation, and concurrent safety.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>