logpuller: split event sink, failure handling, and span lifecycle out of subscription client

[lidezhu]

subscription_client.go currently owns too many responsibilities: event delivery to dynstream, region failure recovery, subscribed-span lifecycle, resolve-lock checking, and slow-region monitoring.

This makes the log puller runtime hard to read, hard to test, and harder to split follow-up changes safely.

We should extract these responsibilities into dedicated components:

• regionEventSink for dynstream push/wake/feedback handling
• regionFailureHandler for region error buffering and retry/reschedule logic
• spanRegistry for subscribed span state, resolve-lock checking, slow-region logging, and related metrics

The goal of this change is structural only:

• keep existing behavior unchanged
• reduce the size and responsibility of subscription_client.go
• make later scheduler/defer refactoring easier and safer