fix: Compute signatures from files so that loki.source.file can handle atomic writes

[kalleep]

PR Description

Noticed when doing some manual testing. Our tailer do not handle atomic writes, e.g. neovim will perform these kind of writes on every save.

Not sure how big of a problem this is in "normal" log rotation environments. What happens before this pr is that if I have a log file in neovim and save :w we will detect a delete event, reopen the file and consume it fully.

If this is something we want to handle we can compute a signature. The signature consist of currently hard coded number of bytes (1024).

This pr handle incomplete signatures and recompute it once we have read past desired size.

Which issue(s) this PR fixes

Notes to the Reviewer

PR Checklist

• Documentation added
• Tests updated
• Config converters updated

[Copilot]

Pull request overview

This pull request adds signature-based detection of atomic writes to the file tailer. The implementation computes a signature from the first N bytes of a file to detect when a file has been replaced atomically (e.g., by editors like neovim). The signature is progressively built as the file grows, with recomputation at specific thresholds (64, 128, 256, 512 bytes) until reaching the target size of 1024 bytes.

Changes:

• Added signature computation and comparison logic to handle atomic file replacements
• Modified file reopening logic to preserve read position when signatures match
• Added progressive signature recomputation as files grow
• Removed automatic offset reset on file deletion, now using signature comparison instead

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 13 comments.

File	Description
internal/component/loki/source/file/internal/tail/signature.go	New file implementing signature struct and methods for computing and comparing file signatures
internal/component/loki/source/file/internal/tail/signature_test.go	Comprehensive unit tests for signature functionality
internal/component/loki/source/file/internal/tail/file.go	Integration of signature computation and comparison into file tailing logic, including initialization, recomputation, and reopen handling
internal/component/loki/source/file/internal/tail/file_test.go	Added atomic write test case and helper function

[kalleep]

@thampiotr I changed to use ReadAt in newSignatureFromFile so we no longer need to perform the extra seek calls we did before.

[kalleep]

Windows test failing is unrelated to this pr TestSourceMapsStoreImpl_CleanOldCachedEntries. Will take a look at fixing that test in another pr.