Add model_request_stream_sync to direct API

[hewliyang]

closes #2096

• sync helper for model_request_stream with some threading overhead
• enforces usage within a context manager to prevent thread leakages

[hyperlint-ai]

PR Change Summary

Added support for synchronous streamed requests in the API documentation.

• Introduced StreamedResponseSync model in the API documentation.
• Added model_request_stream_sync function to the direct API documentation.

Modified Files

• docs/api/models/base.md
• docs/direct.md

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[DouweM]

@hewliyang Nice work, thanks! We'll release this early next week