fix(ai): propagate AbortError in generateText multi-step tool loop

[sleitor]

Summary

Fixes #12878

When an AbortSignal fires between steps in a multi-step tool loop (e.g. during tool execution), some providers return a partial result with finishReason: 'other' instead of throwing. The generateText function silently returned the partial result with no error, giving callers no indication the generation was interrupted.

Root Cause

The do { ... } while (...) loop in generate-text.ts passes the mergedAbortSignal to each model call and tool execution, but it never checks whether the signal was already aborted between iterations. If a provider happens to return rather than throw on an aborted request, the loop continues into the next step as if nothing happened.

Fix

Add an abort guard at the top of each loop iteration:

do {
  // Check if the abort signal was triggered between steps.
  if (mergedAbortSignal?.aborted) {
    throw (
      mergedAbortSignal.reason ??
      new DOMException('This operation was aborted', 'AbortError')
    );
  }
  // ... rest of step
} while (...)

This ensures that regardless of how the provider handled the abort, generateText always throws an AbortError when the signal is fired — consistent with standard Web API abort semantics.

Test

Added a test that:

1. Starts a multi-step generation (model returns a tool call on step 1)
2. Aborts the signal inside the tool's execute function
3. Asserts that generateText throws an AbortError and the second model call is never made

Also updated the existing should forward abort signal to tool execution test: its previous pattern (aborting externally then await generateTextPromise) relied on the function silently absorbing the abort, which is the bug we're fixing. The revised test simply verifies the abort signal is forwarded to the tool execute callback, which is its actual intent.

[sleitor]

👋 Gentle ping — just checking if this is still on the radar for review. Happy to address any feedback or rebase if needed!