test : added unit tests for crypto.ts

[tmdeveloper007]

Closes #2833.

Summary of What Has Been Done: Added vitest unit tests for the token encryption utilities in src/lib/crypto.ts, covering encryptToken, decryptToken, safeCompare, getExpectedSignature, and verifyGitHubSignature.

Changes Made:

• Test encryptToken/decryptToken: AES-256-GCM roundtrip for plain text and unicode, wrong IV length returns null, tampered ciphertext returns null, missing/invalid ENCRYPTION_KEY throws, random IV produces different ciphertexts
• Test safeCompare: equal/different strings, different lengths, empty strings
• Test getExpectedSignature: sha256= prefix, consistency, different secrets/bodies
• Test verifyGitHubSignature: valid signature, wrong secret, tampered body, null signature, missing sha256= prefix

Impact it Made: These functions protect all encrypted tokens (GitHub OAuth, webhook secrets). Vitest test coverage complements the existing node:test suite with proper async isolation and vitest mocking.

[github-actions]

GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

Difficulty (pick one):

• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

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• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

Validation (required to score):

• gssoc:approved — counts for points
• gssoc:invalid / gssoc:spam / gssoc:ai-slop — does not score

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