file_chunk: add stricter checks for broken meta files

[Watson1978]

Which issue(s) this PR fixes:
Fixes #

What this PR does / why we need it:
This PR improves meta file corruption checking.

The meta file contains at least the following field values.

fluentd/lib/fluent/plugin/buffer/file_chunk.rb

Lines 249 to 254 in fa2eb58

	data = @metadata.to_h.merge({
	id: @unique_id,
	s: (update ? @size + @adding_size : @size),
	c: @created_at,
	m: (update ? Fluent::Clock.real_now : @modified_at),
	})

It might be possible that the @size is 0.
@unique_id, @created_at, and @modified_at are set when FileChunk is initialized, so they definitely have some values.
I think these fields should be written in meta file.

So, this PR adds the id, c, and m fields check.

This PR reinforces #1874.

Without this changes, it might causes following error when launch fluentd every time with broken meta file:

2025-06-06 12:11:26 +0900 [error]: unexpected error while checking flushed chunks. ignored. error_class=NoMethodError error="undefined method '<' for nil"
  2025-06-06 12:11:26 +0900 [error]: /Users/watson/src/fluentd/lib/fluent/plugin/output.rb:1479:in 'block in Fluent::Plugin::Output#enqueue_thread_run'
  2025-06-06 12:11:26 +0900 [error]: /Users/watson/src/fluentd/lib/fluent/plugin/buffer.rb:548:in 'block in Fluent::Plugin::Buffer#enqueue_all'
  2025-06-06 12:11:26 +0900 [error]: /Users/watson/src/fluentd/lib/fluent/plugin/buffer.rb:542:in 'Array#each'
  2025-06-06 12:11:26 +0900 [error]: /Users/watson/src/fluentd/lib/fluent/plugin/buffer.rb:542:in 'Fluent::Plugin::Buffer#enqueue_all'
  2025-06-06 12:11:26 +0900 [error]: /Users/watson/src/fluentd/lib/fluent/plugin/output.rb:1479:in 'Fluent::Plugin::Output#enqueue_thread_run'
  2025-06-06 12:11:26 +0900 [error]: /Users/watson/src/fluentd/lib/fluent/plugin_helper/thread.rb:78:in 'block in Fluent::PluginHelper::Thread#thread_create'

Docs Changes:

Release Note:

[Watson1978]

hmm, FileUtils.rm_rf sometimes fails to execute if @bufdir already exists on Windows platform, and it makes the test unstable.

Therefore Dir.mktmpdir is used instead in here.

[Watson1978]

restore_metadata has been invoked only when *.meta file is existed.
This means that write_metadata has been invoked before, and the meta file was created.

In here, it checks the field to be written by write_metadata has the appropriate value.
If the value is not appropriate, it is likely that some kind of error occurred during writing.