feat: isolate async handler execution on dedicated worker event loop

[siwachabhi]

Summary

• Introduces a dedicated persistent worker event loop in a background daemon thread to isolate all async handler execution from the main uvicorn event loop
• Async handlers containing blocking calls (e.g. time.sleep, synchronous HTTP) previously froze /ping health checks, causing container termination — this fix ensures /ping always responds promptly
• Implements three-way handler dispatch: async generators bridged via queue.Queue, regular async via run_coroutine_threadsafe, sync via run_in_threadpool
• Propagates contextvars across event loop boundaries using copy_context() + Django asgiref _restore_context pattern (Python 3.10+ compatible)

Test plan

• 18 new tests in TestWorkerLoopInvocation covering:
  • Async handler runs on worker thread (not main)
  • Sync handler runs in thread pool (worker loop NOT created)
  • Blocking async handler does NOT block /ping
  • Context propagation to async and sync handlers
  • Async generator bridging and streaming
  • Fire-and-forget create_task survives handler return
  • Exception propagation from worker loop
  • Lazy initialization and loop reuse
  • Concurrent async invocations
  • functools.partial and callable class dispatch
  • HEALTHY_BUSY ping status during background tasks
  • End-to-end HTTP tests for sync streaming, async, and async gen streaming
• Updated existing test_async_handler_runs_on_worker_loop to assert worker thread isolation
• All 105 runtime tests pass
• Full test suite: 950 passed, 0 failures
• Pre-commit hooks (ruff lint, ruff format, trailing whitespace) all pass