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Changes being requested

This PR enhances the error handling for aborted requests by propagating the abort reason from the AbortSignal to the APIUserAbortError.

What changed:

• Modified APIUserAbortError instantiation to include the abort signal's reason as the error message
• Updated two locations in client.ts where APIUserAbortError is thrown to pass the abort reason
• Added comprehensive tests to verify:
  • Custom abort reasons are properly propagated to the error
  • Pre-aborted signals prevent requests from being made

Why this is useful:

• Developers can now provide custom abort reasons (e.g., controller.abort('user cancelled')) and receive them in error handlers
• Improves debugging by providing context about why a request was aborted
• Maintains backward compatibility while adding useful functionality

Additional context & links

This change addresses the common use case where applications need to distinguish between different types of request cancellations (user-initiated vs timeout vs navigation away, etc.). By propagating the abort reason, error handlers can make more informed decisions about how to respond to aborted requests.

Example usage:

const controller = new AbortController();
// Abort with a reason
controller.abort('User navigated away');

try {
  await client.get('/foo', { signal: controller.signal });
} catch (error) {
  if (error instanceof APIUserAbortError) {
    console.log(error.message); // "User navigated away"
  }
}