feat: support Moonshot AI Partial Mode for final assistant message prefixes (#19578)

## Background

Moonshot AI consumers can now prefill a response so the model continues
a final assistant-message prefix, enabling guided output and
role-playing scenarios.

## Summary

Added the exported MoonshotAIAssistantMessageProviderOptions type,
serialized partial: true on valid final assistant messages, and added
preflight placement and json_object compatibility validation.

## Testing

Added converter, generate, stream, response-format, invalid-placement,
unchanged-message, and public type-contract tests.

## End-to-end Validation

- Ran the new Partial Mode example against Kimi K3 and reconstructed
`The sky is blue.` from the supplied prefix and returned continuation.
- Live-validated Partial Mode with Moonshot's `json_schema` response
format.

## Documentation

Added Moonshot Partial Mode usage, placement requirements, prefix
concatenation behavior, response-format constraints, and
structured-output parsing guidance under /content.

## Related Issues

Fixes #19545

Co-authored-by: Gregor Martynus <39992+gr2m@users.noreply.github.com>