fix: serialize Moonshot maxOutputTokens using the supported request field (#19605)

## Background

Moonshot requests using the generic maxOutputTokens option sent the
deprecated max_tokens field instead of max_completion_tokens in
streaming and non-streaming calls.

## Root Cause

The shared Moonshot chat request argument builder explicitly mapped
maxOutputTokens to max_tokens; the recorded reproduction and
request-body tests confirmed that both generation paths inherited this
incorrect mapping.

## Summary

Mapped maxOutputTokens to max_completion_tokens, removed
reproduction-only artifacts, and added a patch changeset for
@ai-sdk/moonshotai.

## Testing

Added streaming and non-streaming regression tests covering defined
values, deprecated-field exclusion, and omission when maxOutputTokens is
undefined.

## End-to-end Validation

- `pnpm -C examples/ai-functions exec tsx -e '<fixture-backed
generateText/streamText validation>'` confirmed both paths send only
max_completion_tokens and omit undefined values.
- `pnpm -C examples/ai-functions exec tsx -e '<live Moonshot
generateText/streamText validation>'` completed both live requests
successfully with maxOutputTokens set to 17; each reported length
termination and 17 output tokens.

## Related Issues

Fixes #19556

Closes #19588

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Co-authored-by: ai-sdk-factory <308175966+ai-sdk-factory@users.noreply.github.com>
Co-authored-by: gr2m <39992+gr2m@users.noreply.github.com>