fix: Moonshot structured outputs reject compatible schemas and omit strict validation (#19607)

## Background

Moonshot rejected valid AI SDK structured-output tuple schemas with HTTP
400 because requests used an incompatible schema shape and did not
enable documented strict validation.

## Root Cause

Moonshot response schemas bypassed the existing MFJS normalizer used for
tool schemas, omitted the strict field, and included an undocumented
description field; captured requests and regression tests confirmed
these payload defects.

## Summary

Normalized Moonshot response schemas through the MFJS compatibility
layer, defaulted strict validation to true with a strictJsonSchema
opt-out, removed the undocumented payload member, and updated
documentation, example, and patch changeset.

## Testing

Added request-shape regression coverage for normalization, default
strict mode, strict opt-out, description omission, and unchanged
JSON-object fallback, plus exported provider-option type coverage.

## End-to-end Validation

- `pnpm --filter @ai-sdk/moonshotai build && git show
HEAD:examples/ai-functions/src/reproduction/issue-19544-moonshot-structured-output.ts
| pnpm -C examples/ai-functions exec tsx -` — the live Moonshot
structured-output request succeeded and verified strict opt-out and
JSON-object fallback.

## Related Issues

Fixes #19544

Closes #19592

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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>