feat: support DeepSeek assistant prefix completion (#19402)

## Background

Consumers can prefill a final assistant message and have DeepSeek
continue it through the beta chat-prefix completion API.

## Summary

Added the exported DeepSeekAssistantMessageProviderOptions type,
serialized prefix: true for valid final assistant messages, and rejected
incompatible roles, positions, and non-beta base URLs.

## Testing

Added converter, generate, stream, endpoint-validation, and compile-time
coverage for assistant prefix completion while verifying unchanged
conversion when omitted.

## End-to-end Validation

- Added and successfully ran the DeepSeek assistant-prefix generateText
example against the live beta API.
- Confirmed the provider continued the supplied "The sky is" assistant
prefix.

## Documentation

Added DeepSeek provider documentation covering beta setup, typed message
provider options, placement requirements, proxy behavior, and validation
errors.

## Related Issues

Fixes #19383

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