fix: preserve processed_entries during stale buffer cleanup

[grapestore]

Description

remove_stale_objects in batch-processor-manager.lua deletes stale buffers (empty entry_buffer and batch_to_process) every 1800 seconds. When a buffer is deleted, its processed_entries count is lost.

However, total_pushed_entries lives on the manager instance and accumulates permanently across the worker lifetime. The pending entry count is calculated as total_pushed_entries - total_processed_entries, where total_processed_entries() sums only living buffers' processed_entries.

This mismatch causes pending to grow unboundedly after repeated stale cleanups, eventually exceeding max_pending_entries. Once exceeded, both add_entry and add_entry_to_new_processor reject all new entries — no new processor can be created, so processed_entries stays at 0 permanently. All log entries are dropped until worker restart.

Reproduction (max_pending=10, 3 entries/cycle)

[cycle 1]  push 3 entries → flush complete
           pushed=3, processed=3
           → stale cleanup: buffer deleted → processed=3 lost

[cycle 2]  push 3 entries → new processor created
           pending = 3 - 0 = 3 < 10 → OK
           pushed=6, processed=3 (new buffer)
           → stale cleanup: buffer deleted → processed=3 lost

[cycle 3]  pending = 6 - 0 = 6 < 10 → OK
           pushed=9 → stale cleanup: buffer deleted

[cycle 4]  pending = 9 - 0 = 9 < 10 → OK
           pushed=12 → stale cleanup: buffer deleted

[cycle 5]  pending = 12 - 0 = 12 > 10
           → ❌ permanent drop, no recovery until worker restart

Fix

Add total_stale_processed_entries field to the manager instance. Before deleting a stale buffer, accumulate its processed_entries into this field. Use it as the base value in total_processed_entries().

Changes in 3 places:

1. _M.new — initialize total_stale_processed_entries = 0
2. remove_stale_objects — preserve processed_entries before buffer deletion
3. total_processed_entries — start summation from total_stale_processed_entries

[Baoyuantop]

Please add a test case:

1. Configure max_pending_entries and send several requests to generate processed_entries in the buffer.

2. Trigger a stale cleanup using set_check_stale_interval(1) + ngx.sleep to remove the buffer.

3. Send requests again to verify that the new requests are not incorrectly blocked by max_pending_entries (i.e., processed_entries are correctly preserved).

[Baoyuantop]

Hi @grapestore, please fix the failed CI

[Baoyuantop]

Hi @grapestore, code LGTM, after fixing the failed tests, we can proceed with the merge.