discuss: data lost on worker shut down

### Current Behavior

Any data in the queue is lost when the instance is shutting down or the worker reload is triggered.

1. https://github.com/apache/apisix/blob/master/docs/en/latest/batch-processor.md

### Expected Behavior

Before the APISIX instance shutting down, all left data in the queue should be processed in full and not discarded, unless critical cases (need discussion)

### Error Logs

_No response_

### Steps to Reproduce

No Steps

### Environment

- APISIX version (run `apisix version`): 3.13

Start with docs/en/latest/batch-processor.md to understand the queue and its shutdown behavior. Then trace the worker shutdown and reload lifecycle to identify where queued data is discarded. Done should be defined through the discussion: queued data is processed before shutdown or reload, with any critical-case exceptions explicitly agreed and covered by tests.

Written by the indexing model from the issue text.