refactor(file source): Use Tokio JoinMap

[pront]

Motivation

File cleanup must retain the path when a task fails or panics. Use Tokio's existing JoinMap to handle that bookkeeping, preparing the same approach for the notification-driven file source.

Summary

Replace the cleanup loop's JoinSet and task-ID map with tokio_util::task::JoinMap. Enable join-map on the existing dependency; preserve deletion and error reporting behavior.

References

Preliminary extraction from #26429.

Vector configuration

No configuration changes.

How did you test this PR?

• All 26 file-source and file-source-common tests passed.
• Applied JoinMap to both ifile call sites locally: all 14 E2E tests passed, including truncate/regrow and restart coverage. Both call sites use unique paths.
• Affected-crate Clippy, formatting, and license generation passed. The ifile workspace Clippy check also passed with ifile-tests,sources-file enabled.
• The preliminary branch's workspace Clippy check still encounters master's unchanged unused_async error at src/test_util/http.rs:51.

Does this PR include user facing changes?

• No.

Contributor Guidelines

Followed the repository contribution guidance.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-18T15:18:29.960300Z	e7ab510	New commits
🔒 Security Review	✅ Completed 2026-09-18T15:18:54.778398Z	e7ab510	New commits

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Reviewed commit: fb5fc5f4cb

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