Stage buffer sometimes sticks around and doesn't ever get queued

### Describe the bug

I've been trying to track down what looks like a memory leak for the last week where a stage buffer doesn't get cleared out even though new data arrives. In my latest attempt to isolate the problem, I noticed a jump to 8 MB in the `fluentd_output_status_buffer_stage_byte_size` Prometheus metric, which measures the total bytes of the stage queue:

This jump appears to persist indefinitely until I restart fluentd.

### To Reproduce

I'm still working on this.

### Expected behavior

No memory growth over time.

### Your Environment

```markdown
- Fluentd version: v1.16.5
- Package version: 5.0.4-1
- Operating system: Ubuntu 20.04.6
- Kernel version: 5.15.0-1051-gcp
```

### Your Configuration

I don't have a clear reproduction step yet. Our config looks something like this:

```

@type tail
tag postgres.postgres
path /var/log/postgresql/postgresql.log
pos_file /var/log/fluent/postgres.log.pos
format /(?[^G]*) GMT \[(?\d+), (?\d+)\]: .* user=(?[^,]*),db=(?[^,]*),app=(?[^,]*),client=(?[^ ]*) (?.*)/
time_format %Y-%m-%d %H:%M:%S.%N

@type postgresql_slowlog

@type postgresql_redactor
max_length 200000

@type copy

@type google_cloud
label_map {
"tag": "tag"
}
buffer_type file
buffer_path /opt/fluent/buffers/postgres/google_cloud
buffer_chunk_limit 8MB
buffer_queue_limit 1000
flush_interval 30s
log_level info

@type cloud_pubsub
topic pubsub-postgres-inf-gprd
project my-project
buffer_type file
buffer_path /opt/fluent/buffers/postgres/cloud_pubsub
buffer_chunk_limit 8MB
buffer_queue_limit 1000
flush_interval 30s

```

### Your Error Log

The stuck 8MB buffer seems to have coincided with an EOF error:

```shell
2024-10-08 10:40:21 +0000 [error]: #0 /var/lib/fluent/vendor/bundle/ruby/3.2.0/gems/fluentd-1.16.5/lib/fluent/plugin/buffer/file_chunk.rb:170:in `open'
2024-10-08 10:40:21 +0000 [error]: #0 /var/lib/fluent/vendor/bundle/ruby/3.2.0/gems/fluentd-1.16.5/lib/fluent/event.rb:318:in `each'
2024-10-08 10:40:21 +0000 [error]: #0 /etc/fluent/plugin/out_cloud_pubsub.rb:62:in `write'
2024-10-08 10:40:21 +0000 [error]: #0 /var/lib/fluent/vendor/bundle/ruby/3.2.0/gems/fluentd-1.16.5/lib/fluent/compat/output.rb:131:in `write'
2024-10-08 10:40:21 +0000 [error]: #0 /var/lib/fluent/vendor/bundle/ruby/3.2.0/gems/fluentd-1.16.5/lib/fluent/plugin/output.rb:1225:in `try_flush'
2024-10-08 10:40:21 +0000 [error]: #0 /var/lib/fluent/vendor/bundle/ruby/3.2.0/gems/fluentd-1.16.5/lib/fluent/plugin/output.rb:1538:in `flush_thread_run'
2024-10-08 10:40:21 +0000 [error]: #0 /var/lib/fluent/vendor/bundle/ruby/3.2.0/gems/fluentd-1.16.5/lib/fluent/plugin/output.rb:510:in `block (2 levels) in start'
2024-10-08 10:40:21 +0000 [error]: #0 /var/lib/fluent/vendor/bundle/ruby/3.2.0/gems/fluentd-1.16.5/lib/fluent/plugin_helper/thread.rb:78:in `block in thread_create'
2024-10-08 10:40:21 +0000 [error]: #0 failed to purge buffer chunk chunk_id="623f4c358bd4b7cd7f63a4eb7410b459" error_class=Errno::ENOENT error=#
2024-10-08 10:40:21 +0000 [error]: #0 /var/lib/fluent/vendor/bundle/ruby/3.2.0/gems/fluentd-1.16.5/lib/fluent/plugin/buffer/file_chunk.rb:161:in `unlink'
2024-10-08 10:40:21 +0000 [error]: #0 /var/lib/fluent/vendor/bundle/ruby/3.2.0/gems/fluentd-1.16.5/lib/fluent/plugin/buffer/file_chunk.rb:161:in `purge'
2024-10-08 10:40:21 +0000 [error]: #0 /var/lib/fluent/vendor/bundle/ruby/3.2.0/gems/fluentd-1.16.5/lib/fluent/plugin/buffer.rb:601:in `block in purge_chunk'
2024-10-08 10:40:21 +0000 [error]: #0 /opt/fluent/lib/ruby/3.2.0/monitor.rb:202:in `synchronize'
2024-10-08 10:40:21 +0000 [error]: #0 /opt/fluent/lib/ruby/3.2.0/monitor.rb:202:in `mon_synchronize'
2024-10-08 10:40:21 +0000 [error]: #0 /var/lib/fluent/vendor/bundle/ruby/3.2.0/gems/fluentd-1.16.5/lib/fluent/plugin/buffer.rb:592:in `purge_chunk'
2024-10-08 10:40:21 +0000 [error]: #0 /var/lib/fluent/vendor/bundle/ruby/3.2.0/gems/fluentd-1.16.5/lib/fluent/plugin/output.rb:1110:in `commit_write'
2024-10-08 10:40:21 +0000 [error]: #0 /var/lib/fluent/vendor/bundle/ruby/3.2.0/gems/fluentd-1.16.5/lib/fluent/plugin/output.rb:1229:in `try_flush'
2024-10-08 10:40:21 +0000 [error]: #0 /var/lib/fluent/vendor/bundle/ruby/3.2.0/gems/fluentd-1.16.5/lib/fluent/plugin/output.rb:1538:in `flush_thread_run'
2024-10-08 10:40:21 +0000 [error]: #0 /var/lib/fluent/vendor/bundle/ruby/3.2.0/gems/fluentd-1.16.5/lib/fluent/plugin/output.rb:510:in `block (2 levels) in start'
2024-10-08 10:40:21 +0000 [error]: #0 /var/lib/fluent/vendor/bundle/ruby/3.2.0/gems/fluentd-1.16.5/lib/fluent/plugin_helper/thread.rb:78:in `block in thread_create'
2024-10-08 10:40:21.470273004 +0000 fluent.error: {"chunk_id":"623f4c358bd4b7cd7f63a4eb7410b459","error_class":"Errno::ENOENT","error":"#","message":"failed to purge buffer chunk chunk_id=\"623f4c358bd4b7cd7f63a4eb7410b459\" error_class=Errno::ENOENT error=#","tag":"fluent.error","environment":"gprd","hostname":"example.com","fqdn":"example.com","stage":"main","shard":"backup","tier":"db","type":"patroni"}
```

### Additional context

Note that previously when log messages were up to 3 MB, I would see more of these "step" jumps in memory usage. I've altered our filters to truncate the log messages to 200K, which seems to have stopped most of these stage buffer leaks. But I'm still wondering if there is a corner case here where the file buffer got cleared but the stage buffer did not.