fix: prevent completed workflow harness turns from being suspended at time-slice deadlines (#19432)

## Background

Completed harness turns could fail at the workflow time-slice boundary
with a "no unfinished turn to suspend" error while their UI stream was
still closing.

## Root Cause

The time-slice timer called suspendTurn() unconditionally, using stream
completion instead of the session lifecycle state. The deterministic
reproduction confirmed the timer fired after hasUnfinishedTurn() became
false.

## Summary

Guarded timer-driven suspension with hasUnfinishedTurn(), added a patch
changeset, and removed reproduction-only artifacts.

## Testing

Added deterministic regression coverage for a completed turn with an
open stream, asserting normal completion, stream closure, detach
cleanup, and zero suspension calls.

## End-to-end Validation

- `pnpm -C examples/ai-functions exec tsx
src/reproduction/issue-19419-workflow-harness-time-slice.ts` exited
successfully with all terminal assertions passing.

## Related Issues

Fixes #19419

Closes #19422

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Co-authored-by: ai-sdk-factory <308175966+ai-sdk-factory@users.noreply.github.com>
Co-authored-by: jdanielnd <571693+jdanielnd@users.noreply.github.com>
Co-authored-by: Felix Arntz <felix.arntz@vercel.com>