Changes:
- Make JWT validator lazy-load JWKS on first use (not on init)
- Thread-safe JWKS loading with mutex
- Fixes test failures (JWKS 404 was panicking on NewValidator)
- Add unit tests for JWT validation logic
Tests now pass:
✅ Check permissions (sqs:read, sqs:write, wildcard)
✅ Reject empty/invalid/malformed tokens
✅ Handle missing permissions claim
All 100% passing with no external dependencies.
