Fix simulator errors when sending message after flow completes

[Copilot]

The simulator was throwing TypeErrors when users attempted to send messages after a flow had completed. This occurred due to two null pointer exceptions:

Issues Fixed

1. Cannot read properties of null (reading 'unshift') (Line 343)

When a flow completes, the server response may return runContext.events as null or undefined. The simulator was attempting to call unshift() on this null value when adding the user's message event:

// Before - would throw if runContext.events is null/undefined  
if (msgEvt) {
  runContext.events.unshift(msgEvt);
}

// After - safely handles null/undefined events
if (!runContext.events) {
  runContext.events = [];
}
if (msgEvt) {
  runContext.events.unshift(msgEvt);
}

2. Cannot read properties of undefined (reading 'status') (Line 521)

Network errors and other error scenarios could result in an error object without a response property. The error handler was accessing error.response.status without checking if response existed:

// Before - would throw if error.response is undefined
.catch(error => {
  if (error.response.status) {
    // ...
  }
  const text = error.response.status > 499 
    ? 'Server error, try again later'
    : error.response.data.error;
})

// After - safely handles missing response
.catch(error => {
  if (error.response && error.response.status) {
    // ...
  }
  const text = error.response && error.response.status > 499
    ? 'Server error, try again later'
    : error.response && error.response.data && error.response.data.error
    ? error.response.data.error
    : 'An error occurred';
})

Testing

Added comprehensive tests covering:

• Null and undefined events arrays in updateRunContext
• Error handling scenarios with and without response objects
• All existing tests continue to pass (564 tests across 102 test suites)

The fixes are minimal and surgical, maintaining existing behavior while preventing runtime exceptions in edge cases.

Fixes #1332.

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