Add comprehensive unit tests for error classes

[taralamia]

What's in this PR?

This PR adds comprehensive unit tests for the OAuthError and TimeoutError classes in src/errors.ts, addressing issue #3.

What was tested?

The tests cover all requirements from the issue:
✅ Constructor property assignment
✅ Error inheritance from native Error class
✅ Default message fallbacks
✅ Correct error naming (OAuthError, TimeoutError)
✅ Stack trace maintenance
✅ JSON serialization behavior
✅ Edge cases (long strings, unicode, null/undefined values)
✅ Error comparison (instance vs. value equality)
✅ Error recovery (verified no implementation in current design)

Key Findings (Please Read)

All tests pass except for the JSON serialization test. This test reveals the current behavior:

• OAuthError: Only the custom properties (error, error_description, error_uri) are serialized. The name and message properties from the inherited Error class are omitted because they are non-enumerable.
• TimeoutError: Serializes to an empty object {} for the same reason.
  This is the current, tested behavior. If this is not the desired behavior for the library (e.g., if errors should serialize fully for logging), a fix would require implementing a toJSON() method on these classes. This could be explored in a future issue.

Notes

• I followed the suggested test structure using bun:test.
• The tests confirm the core error-handling logic is solid and works as intended.
• Utility functions were created for robust error comparison testing (sameErrorInstance, equalErrorByValue, equalErrorStrict).

[koistya]

Hey @taralamia! Thanks for getting this PR started. I've pushed some improvements to help get it ready for merge:

Fixed issues:

• Line 64 was creating the wrong error type (OAuthError instead of TimeoutError) - fixed ✅
• Removed the duplicate stack trace test that was causing confusion
• Cleaned up the NPM lock file that snuck in (we're using Bun)

Enhanced the tests:

• Expanded from 10 to 23 tests with much better coverage
• Added the missing JSON serialization tests you mentioned
• Included edge cases for empty strings, long strings, and unicode
• Added error comparison tests to verify instance uniqueness
• Reorganized everything with nested describe blocks for clarity

All tests are passing now! The coverage looks solid and addresses everything from issue #3. Nice work on the initial implementation - the foundation was good, just needed some polish.

Let me know if you want to review any of the changes!

[taralamia]

Thanks @koistya I really appreciate you polishing up the PR and adding those comprehensive tests. And I’m happy to contribute since I got to learn more about Bun in the process.