fix: include parsed structured output in streamText end callbacks (#17717)

## Background

The `streamText` structured-output replacement omitted the complete
parsed output from `onEnd`/`onFinish`, preventing callback-based
persistence workflows supported by `streamObject`.

## Root Cause

`streamText` assembled end events exclusively from step results, while
complete output parsing was only exposed through `result.output`.

## Summary

- expose schema-inferred optional `output` in `onEnd`/`onFinish`
- cache complete-output parsing so callbacks and `result.output` share
one parse
- keep `result.output` settlement independent from arbitrary callback
completion
- export `StreamTextEndEvent` and `StreamTextOnEndCallback`
- keep telemetry events restricted to their existing shape
- update docs, reproduction, focused tests, and patch changeset

Callers that need to wait for asynchronous callback work can await
`result.consumeStream()`.

## Testing

- callback output and schema inference
- reusable public callback/event types
- single complete-output parse
- parse failures return `undefined` to the callback while
`result.output` rejects
- `result.output` remains independent from active callbacks
- callbacks can await `result.output` after asynchronous work
- full `ai` Node and Edge suites
- workspace type checking, linting, formatting, and package build
- provider-independent reproduction

## Related Issues

Fixes #11418

Closes #17701

Co-authored-by: pltoledo <57258959+pltoledo@users.noreply.github.com>

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Co-authored-by: ai-sdk-factory <308175966+ai-sdk-factory@users.noreply.github.com>
Co-authored-by: pltoledo <57258959+pltoledo@users.noreply.github.com>
Co-authored-by: Gregor Martynus <39992+gr2m@users.noreply.github.com>
Co-authored-by: ai-sdk-factory[bot] <305873210+ai-sdk-factory[bot]@users.noreply.github.com>