chore(backend): add contract tests for errorHandler mapping to http status codes

[Joeloo1]

Description

Implement comprehensive contract tests for the errorHandler middleware to ensure correct mapping of error classes to HTTP status codes. This PR also includes security enhancements and robustness improvements for error identification.

Closes #258

Changes

• Security: Masked internal error messages in production for non-AppError types in src/middleware/errorHandler.ts.
• Tests: Extended src/middleware/errorHandler.test.ts with explicit contract tests for all error subclasses and a production environment suite.
• Robustness: Improved isAppError in src/errors/index.ts to use a brand property, ensuring consistency across module instances.

Test Output

PASS src/middleware/errorHandler.test.ts
Error Handler
✓ should handle AppError with correct response shape
✓ should handle generic Error with default values
...
✓ map ForbiddenError to 403
✓ map NotFoundError to 404
✓ map PaymentRequiredError to 402
✓ map TooManyRequestsError to 429
Error Handler - Production Environment
✓ should mask generic error message in production
✓ should NOT mask AppError messages in production

Test Suites: 1 passed, 1 total
Tests: 12 passed, 12 total

Security & Data Integrity Notes

• Assumption: AppError subclasses are intended for user-facing messages and are safe to expose in production.
• Assurances: Generic errors are now masked with a generic message in production to prevent leakage of infrastructure details. Full stack traces are preserved in server-side logs for debugging.

[drips-wave]

@Joeloo1 Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits