fix(ui): prevent TypeError in onFinish when activeResponse is undefined

[codewithkenzo]

Background

When using useChat with onFinish callback and resume enabled, users encounter:

TypeError: Cannot read properties of undefined (reading 'state')

This happens because this.activeResponse in the finally block can be:

1. Undefined - if an error is thrown before assignment in the try block
2. Overwritten - by concurrent makeRequest calls

Summary

Use local variable capture pattern to safely reference activeResponse in the finally block:

• Declare let activeResponse before try block
• Assign to local variable first, then to this.activeResponse
• Check local variable in finally block before calling onFinish

This is a well-established pattern for try-finally resource management (see TypeScript #28153).

Manual Verification

Build passes locally. The fix is minimal and follows the exact pattern suggested by @codybrouwers in the issue comments, which multiple users confirmed works via patch-package.

Checklist

• Tests have been added / updated (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)
• A patch changeset for relevant packages has been added (for bug fixes / features - run pnpm changeset in the project root)
• I have reviewed this pull request (self-review)

Related Issues

Fixes #8477

Thanks to @codybrouwers for identifying the root cause and providing the solution.