Add unit tests for webhook-utils signature verification

[bridgetthnkechi87-cloud]

Closes #293

Summary

• Added additional edge-case tests for webhook signature verification
• Covered duplicate v1 signature values, whitespace-only signatures, byte-exact payload verification, and timestamp tolerance boundaries
• Kept the existing Node built-in test runner
• Did not change webhook verification source logic

Testing

• pnpm --filter @guildpass/webhook-utils test
  • Result: 40/40 tests passed
• pnpm --filter @guildpass/webhook-utils typecheck
  • Result: passed

Notes

• All fixtures use fixed test-only secrets and payloads.
• No real or production secrets are used.