API mock interceptor: XHR onreadystatechange wrapper creates synthetic Event

[tyevco]

Summary

In plugins/api-mock-interceptor/src/core/interceptor.ts (lines 236-295), the XHR onreadystatechange wrapper calls the original handler with a newly constructed Event object instead of forwarding the actual event.

Current Behavior

const originalOnReadyStateChange = xhr.onreadystatechange;
xhr.onreadystatechange = function() {
  // ... interceptor logic ...
  
  if (originalOnReadyStateChange) {
    return originalOnReadyStateChange.call(this, new Event('readystatechange'));
  }
};

Problem

The original handler receives a synthetic new Event('readystatechange') instead of the real browser event. This means:

• event.target will be null instead of the XHR object
• event.currentTarget will be null
• Any custom properties the browser adds to the event are lost
• Code that inspects event.target to get the XHR reference will break

Suggested Fix

Capture and forward the actual event:

xhr.onreadystatechange = function(event: Event) {
  // ... interceptor logic ...
  
  if (originalOnReadyStateChange) {
    return originalOnReadyStateChange.call(this, event);
  }
};

Priority

Low-Medium — most XHR handlers use this (which is correctly bound) rather than event.target, but it's a correctness issue.