[maxx-GENIUS]

Summary

This PR adds comprehensive unit test coverage for backend/src/utils/asyncHandler.ts ensuring that unhandled rejections occurring after response headers have been sent (res.headersSent = true) are properly caught and forwarded to next(), while asserting complete normalization contracts across non-Error rejections.

Closes #2090

Key Test Coverage Added

1. Rejected Promises After Headers Are Sent:
   • Catches and forwards rejected promise when headers are committed by res.writeHead().
   • Catches and forwards deferred background rejections after res.send().
   • Normalizes non-Error rejections thrown after res.headersSent is set to true.
   • Handles promises rejecting when res.headersSent is already true upon entry.
2. Error Forwarding & Property Preservation:
   • Forwards genuine Error instances as-is to next(err).
   • Preserves custom properties on errors (such as statusCode, errorCode).
   • Preserves error subclasses (TypeError, RangeError, etc.) without mutation.
3. Non-Error Rejection Normalization:
   • Asserts string rejections normalize to Error('Non-Error rejection: <val>') with cause attached.
   • Asserts number, boolean, null, undefined, plain object, and symbol rejections normalize predictably with root value preserved in err.cause.
4. Execution Flow Contracts:
   • Catches synchronously thrown errors inside async handlers.
   • Ensures successful async functions do not call next(err).
   • Supports explicit next() invocations in middleware chains.
   • Verifies faithful forwarding of req, res, and next references.

Verification

• npm test -- src/utils/__tests__/asyncHandler.test.ts (18/18 tests passing, 100% green)
• npm run build (0 TypeScript compilation errors)

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