test : added unit tests for crypto safeCompare and verifyGitHubSignature

[tmdeveloper007]

Refs #1274.

Summary of What Has Been Done:
Added comprehensive unit tests for the safeCompare and verifyGitHubSignature functions in src/lib/crypto.ts. Tests cover empty strings, different length comparisons, valid/invalid HMAC signatures, null signature handling, and signature prefix validation.

Changes Made:

• Enhanced test/crypto.test.js with 8 new test cases:
  • safeCompare with identical strings
  • safeCompare with different length strings
  • safeCompare with non-identical same-length strings
  • safeCompare with empty strings
  • safeCompare with empty vs non-empty
  • verifyGitHubSignature with valid signature
  • verifyGitHubSignature with invalid signature
  • verifyGitHubSignature with null signature
  • verifyGitHubSignature without sha256= prefix

Impact it Made:
Improved test coverage for security-critical cryptographic utilities. All 12 tests pass.

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