feat(middleware): sanitize sensitive fields from error responses (#4)

[Cid-oe]

Summary

Closes #4.

Adds a pure, structured error-response sanitizer helper module in src/middleware/errorResponse.ts and integrates it into errorHandler to prevent sensitive fields (credentials, private keys, database connection strings, tokens, etc.) from leaking in error responses.

Changes

• Created src/middleware/errorResponse.ts exporting:
  • DENY_LISTED_KEYS: Set of sensitive keys to strip (case-insensitively).
  • sanitizeErrorDetails: Pure recursive function removing sensitive keys from objects and arrays, while passing primitives and nullish values safely through.
• Exported the module from src/middleware/index.ts.
• Updated src/middleware/errorHandler.ts to pass details and object development error.stack through sanitizeErrorDetails.
• Created comprehensive unit test suite in tests/unit/middleware/errorResponse.test.ts covering:
  • Top-level deny-listed keys (case-insensitive)
  • Deeply nested deny-listed keys
  • Arrays containing sensitive keys
  • Primitives and non-object inputs (null, undefined, strings, numbers)
  • Complex nested structures

Verification

• pnpm test: 9/9 tests pass
• pnpm typecheck: zero type errors
• pnpm format:check: all files format compliant
• pnpm lint: 0 errors