feat(client): propagate AbortSignal reason to APIUserAbortError

[christian-bromann]

• I understand that this repository is auto-generated and my pull request may not be merged

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"
  }
}