test: add unit tests for token expiry validator (#769)Feature/token expiry validator

[yugomania]

Closes #769

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Summary

Implement and test token expiry validation based on the token's iat claim and configured TTL.

What Changed

• Added validateTokenExpiry to calculate the expected expiry timestamp as iat + ttlSeconds.
• Added TOKEN_EXPIRY_TAMPERED and MISSING_IAT error codes.
• Added unit tests covering all required token expiry validation scenarios.
• Documented the new error codes in ERROR_CODE_REGISTRY.md.

Acceptance Criteria

• Correct exp === iat + ttl passes validation.
• Extended exp > iat + ttl fails with token_expiry_tampered.
• Shortened exp < iat + ttl fails with token_expiry_tampered.
• Missing iat fails with missing_iat.

Testing

• pnpm test -- token-expiry → 4 tests passed
• pnpm run lint → Passed
• pnpm run build → Passed

Files Changed

• src/constants/error.constants.ts
• src/utils/token-expiry.validator.ts
• src/utils/token-expiry.validator.test.ts
• docs/ERROR_CODE_REGISTRY.md

Scope

This contribution is limited to token expiry validation, its required error codes, tests, and corresponding documentation. No unrelated functionality was modified.

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