feat: support names on DeepSeek system, user, and assistant messages (#19399)

## Background

DeepSeek users can identify participants in multi-party conversations by
attaching provider-specific names to individual messages.

## Summary

Added the exported DeepSeekMessageProviderOptions type and serialization
of providerOptions.deepseek.name for system, user, and assistant
messages. Invalid values are rejected, tool-message names are ignored
with an unsupported warning, and unset names remain omitted.

## Testing

Added converter, generate, stream, and compile-time tests covering
supported roles, multimodal and tool-call content, default omission,
invalid values, and unsupported tool-message placement.

## End-to-end Validation

- Added and successfully ran the DeepSeek message-names generateText
example against the live API.
- Built the DeepSeek package and confirmed the new public type is
present in generated declarations.

## Documentation

Added DeepSeek provider documentation with typed generateText and
streamText examples, supported-role behavior, validation details,
tool-message limitations, and privacy guidance.

## Related Issues

Fixes #19385

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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>