feat: support names on Moonshot AI system, user, and assistant messages (#19580)

## Background

Moonshot AI consumers can now identify participants in multi-party
conversations by attaching provider-specific names to individual
messages.

## Summary

Added typed and exported MoonshotAIMessageProviderOptions, validated
per-message options, serialized supported message names, and emitted
warnings for unsupported tool-message names.

## Testing

Added converter, generate, stream, and compile-time tests covering
supported roles, multimodal and tool-call content, omission, invalid
values, custom namespaces, and tool-message warnings.

## End-to-end Validation

- Built the Moonshot AI package, confirmed the generated declaration
export, and successfully ran the new message-names generateText example
against the live Kimi K3 API.

## Documentation

Added Moonshot AI provider documentation with typed generateText and
streamText examples, supported roles, validation and omission behavior,
tool-message limitations, and privacy guidance.

## Related Issues

Fixes #19550

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