fix: missing streamed reasoning for Open Responses reasoning summary text events (#19661)

## Background

OpenAI-compatible Responses streams using the standard reasoning summary
carrier emitted reasoning start and end events but omitted the streamed
text, leaving callers with empty reasoning until final metadata arrived.

## Root Cause

The Open Responses transform handled the LM Studio
`response.reasoning_text.delta` extension but omitted the standard
`response.reasoning_summary_text.delta` event even though it was already
in the event union; the regression fixture and immutable reproduction
confirmed the events parsed successfully but were silently discarded.

## Summary

The transform now maps standard reasoning summary text deltas to
`reasoning-delta` parts using `item_id` while preserving extension
support and authoritative final summary metadata. The pull request
includes a representative SSE fixture, focused regression coverage, and
a patch changeset, with no dependency or configuration changes.

## Testing

Regression coverage verifies reasoning start, two streamed summary
deltas, reasoning end, and final summary metadata. All 89 Open Responses
tests passed in both Node and Edge environments, full repository type
checking passed, formatting and lint checks passed, and the immutable
reproduction no longer reproduces the issue.

## End-to-end Validation

- `pnpm -C examples/ai-functions exec tsx
src/reproduction/issue-19659-open-responses-reasoning-summary.ts` exited
0, streamed `Thinking.` through two reasoning deltas, matched final
summary metadata, and was classified `no-longer-reproduces`.

## Related Issues

Fixes #19659

Closes #19660

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Co-authored-by: ai-sdk-factory <308175966+ai-sdk-factory@users.noreply.github.com>
Co-authored-by: Astro-Han <255364436+Astro-Han@users.noreply.github.com>