enhancement(buffers): improve disk buffer lifecycle visibility

[Jansen-w]

Summary

• Report disk buffers under data_dir that are not referenced by the active configuration.
• Run discovery on one dedicated worker outside Tokio, bounded to one active scan and one latest pending request, so filesystem traversal cannot delay startup, reload, or runtime shutdown.
• Suppress stale scan results across rapid reloads and resolve symlinked roots, directories, and files without following cycles or paths outside the buffer root.
• Schedule orphan diagnostics only after successful startup or reload. The immediate post-reload scan temporarily excludes paths from the previous authoritative configuration because removed sinks may still be draining them.
• Log when existing disk buffers contain unread data and report approximate unread event/byte progress while removed sinks drain in the background.

Graceful shutdown behavior is intentionally unchanged here; that behavior is isolated in #26238.

Vector configuration

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

How did you test this PR?

• Disk-buffer encodability, filtering, overflow, and recovered-usage tests
• Seven orphan-discovery tests covering configured paths, nested IDs, symlinked roots/files/directories, cycle avoidance, root escapes, stale-result suppression, request coalescing, and non-blocking scanner scheduling/drop
• Disk-buffer topology reload regressions
• Real disk-buffer drain test covering nonzero usage, acknowledgement, monitor completion, and lock release/reopen
• Combined enhancement(buffers): improve disk buffer lifecycle visibility #26050/fix(topology): await removed disk sinks on shutdown #26238 topology suite: 65 passed, 1 ignored
• cargo clippy -p vector-buffers --all-targets -- -D warnings
• cargo clippy -p vector --lib --tests -- -D warnings
• cargo fmt --all -- --check
• Full GitHub Actions test, clippy, formatting, generated-doc, and changelog checks

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

Related: #26238