fix(ui): add missing optional chaining for activeResponse in makeRequest finally block

[krislavten]

Bug

When two makeRequest calls run concurrently (e.g. sendMessage + resumeStream), the first to complete sets this.activeResponse = undefined in its finally block. The second call's finally block then crashes:

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

This is caused by an inconsistency in packages/ai/src/ui/chat.ts — the onFinish callback in the finally block uses a non-null assertion (!) for message but optional chaining (?.) for finishReason:

// Before (broken)
this.onFinish?.({
  message: this.activeResponse!.state.message,     // ← non-null assertion crashes
  // ...
  finishReason: this.activeResponse?.state.finishReason,  // ← has ?. but .state doesn't
});

Fix

• Replace this.activeResponse!.state.message with this.activeResponse?.state?.message
• Add ?. to .state?.finishReason for full safety
• Update ChatOnFinishCallback type to allow message: UI_MESSAGE | undefined

// After (safe)
this.onFinish?.({
  message: this.activeResponse?.state?.message,
  // ...
  finishReason: this.activeResponse?.state?.finishReason,
});

Affected Versions

This bug exists from at least 6.0.x through the current 7.0.0-beta — any version with the AbstractChat class and its makeRequest method.

Reproduction

1. Create a chat instance with onFinish callback
2. Trigger two concurrent makeRequest calls (e.g. call sendMessage() and resumeStream() without awaiting the first)
3. The first to resolve sets this.activeResponse = undefined
4. The second hits the finally block → TypeError

Test Plan

• All 26 existing tests in chat.test.ts pass
• Full monorepo build succeeds (69/69 tasks)
• Lint (ultracite) passes via pre-commit hook