fix(ai): skip stringifying text when streaming partial text

[aayush-kapoor]

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

repro

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`,
  );
});

Checklist

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

Related Issues

fixes #13839