## Background

#13839

streamText with the default text output called JSON.stringify on the
full accumulated text on every single streaming chunk that creates
increasingly large string copies per stream casuing memory issues

## Summary

- skip `JSON.stringify` when the partial output is already a string
- structured outputs still go through stringify as before since they
need serialization to compare.
- compare the text directly
- no extra full-string serialization per chunk

## Manual Verification

tried reproducing via 
<details>
<summary>repro</summary>

```ts
import { openai } from '@ai-sdk/openai';
import { streamText } from 'ai';
import { run } from '../../lib/run';

run(async () => {
  const result = streamText({
    model: openai.responses('gpt-4o-mini'),
    prompt:
      'Write an extremely detailed 5000-word essay about the history of computing. Include every detail you can.',
  });

  let chunks = 0;
  for await (const textPart of result.textStream) {
    chunks++;
    if (chunks % 100 === 0) {
      const mb = (process.memoryUsage().heapUsed / 1024 / 1024).toFixed(1);
      console.log(`chunk ${chunks} — heap: ${mb}MB`);
    }
  }

  console.log(`\nTotal chunks: ${chunks}`);
  console.log(
    `Final heap: ${(process.memoryUsage().heapUsed / 1024 / 1024).toFixed(1)}MB`,
  );
});
```
</details>

## Checklist

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

## Related Issues

fixes #13839