Fix Python 3.14 asyncio event loop crash in Mangum webhook handler

[hiroshinishio]

Summary

• Python 3.14 removed implicit event loop creation in asyncio.get_event_loop(), causing Mangum's HTTPCycle.__call__ to crash with RuntimeError on every webhook invocation
• 3027 out of 6462 CloudWatch log events in the last 2 hours were this crash — all webhook processing was broken
• Schedule triggers (EventBridge) worked because they bypass Mangum; webhook events (GitHub PR opened, push, etc.) go through Mangum and crashed
• Fix: ensure an event loop exists in handler() before calling mangum_handler(), same pattern Mangum uses at init time but executed at request time
• Even Mangum 0.21.0 (latest) has the same bug — upgrading won't help