test : added unit tests for crypto-edge.ts

[tmdeveloper007]

Closes #2829.

Summary of What Has Been Done: Added vitest unit tests for the edge-compatible AES-256-GCM decryption wrapper in src/lib/crypto-edge.ts.

Changes Made:

• Test: invalid or missing ENCRYPTION_KEY env var returns null
• Test: non-hex encrypted string returns null
• Test: odd-length encrypted string returns null
• Test: non-hex IV returns null
• Test: wrong IV length returns null
• Test: tampered ciphertext returns null
• Test: hex with invalid characters returns null

Impact it Made: This module protects edge runtime token decryption. Test coverage ensures malformed inputs are handled safely without throwing.

[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

Quality (optional):

• 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

Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus