feat: add validated DeepSeek user ID chat option (#19397)

## Background

DeepSeek consumers need to associate chat requests with an opaque
end-user identifier for content-safety tracing and request isolation.

## Summary

Added the optional `userId` DeepSeek chat provider option, validated its
documented character set and 512-character limit, and serialized valid
values as `user_id` without changing omitted-option requests.

## Testing

Added generate and stream request-body coverage, omission and
invalid-value validation tests, and compile-time coverage for the
exported optional string type.

## End-to-end Validation

- Added and successfully ran the new DeepSeek `userId` generate-text
example against the live API.

## Documentation

Updated the DeepSeek provider documentation with `userId` usage,
validation constraints, isolation purposes, and privacy guidance.

## Related Issues

Fixes #19380

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