enhancement(buffers): improve disk buffer lifecycle visibility

[Jansen-w]

Summary

• Warn when the disk buffer root contains a Vector disk buffer that is not referenced by the loaded configuration, including its inferred orphaned_buffer_id.
• Log when an existing disk buffer is opened or reused during configuration reload.
• Track approximate unread event and byte counts so background sink drains can report progress and completion.
• Preserve existing reload failure propagation while adding drain visibility.

Vector configuration

No standalone configuration is required. The behavior applies to sinks configured with buffer.type: disk during startup and configuration reload.

How did you test this PR?

• cargo test -p vector --lib config::validation::tests
• cargo test -p vector --lib topology::running::disk_buffer_drain_tests
• cargo test -p vector --lib topology::test::reload
• cargo test -p vector-buffers initial_size_correct_with_multievents
• cargo clippy -p vector -p vector-buffers --lib -- -D warnings
• cargo vdev check changelog-fragments

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. A changelog fragment is included.
• No. A maintainer will apply the no-changelog label to this PR.

References

None.