test: add unit test coverage for attestation path validation

[corepacket]

Summary

Fixes #20
This PR adds unit test coverage for the attestation path validation logic introduced in #18.

In PR #18, we introduced a validation mechanism to detect unexpected files in attestations (e.g., .git directories and *.log files) and surface them as warnings without modifying the original data.

This PR builds on that work by adding tests to ensure the validation behaves correctly.

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What’s Added

Unit Tests for matchesPattern

• Covers detection of:
  • .git directories (including nested paths)
  • *.log files
• Includes negative cases:
  • normal source files
  • documentation files
  • edge cases like empty paths and similar-looking paths (mygit/...)

Unit Tests for ValidatePath

• Verifies that:
  • Suspicious paths correctly trigger warnings
  • Safe paths do not produce any warnings
• Uses stderr capture to validate warning output behavior

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Why This Change

• Improves confidence in validation logic introduced in feat: Attestation Path Validation for Unexpected Files #18
• Ensures correct behavior across different path structures
• Prevents regressions in future changes

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Notes

• Uses standard Go testing package (no additional dependencies)
• Keeps tests isolated from implementation changes
• Focuses on behavior validation rather than implementation details