Improved checksum parsing

[bdehamer]

Per: actions/attest-build-provenance#454 (comment)

Improve parsing of checksums files to make more tolerant of different formats.

Most tools which generate checksums follow the guidance for the GNU md5sum tool:

For each file, we print the checksum, a space, a flag indicating binary or text input mode, and the file name. Binary mode is indicated with ‘*’, text mode with ‘ ’ (space). Binary mode is the default on systems where it’s significant, otherwise text mode is the default.

However, not all tools include the input mode flag. This change will updating the parsing logic to make it tolerant of tools which don't include the input mode flag.

[Copilot]

Pull Request Overview

This PR improves the tolerance of checksum file parsing by making the input mode flag character optional, addressing compatibility issues with tools that don't include the binary/text mode indicator.

Key changes:

• Updated parsing logic to handle checksums with or without the mode flag character ('*' or ' ')
• Added comprehensive test coverage for the new parsing behavior

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/subject.ts	Modified checksum parsing to optionally handle mode flag characters
tests/subject.test.ts	Added test cases for checksums with and without mode flags

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