fix: map SSE error types to correct HTTP status codes in streaming

[Jah-yee]

Summary

When the API returns HTTP 200 (streaming started fine) but then sends an SSE error event like overloaded_error, the SDK was creating an APIStatusError with status_code=200 instead of the correct error code (e.g., 529 for overloaded).

This happened because _streaming.py passed the original HTTP response (status 200) to _make_status_error, which then selected the error subclass based on that 200 status code.

Fix

Added a mapping from SSE error types to HTTP status codes, and create a mock response with the correct status code when handling SSE errors in both sync and async streaming.

Changes

• Added _SSE_ERROR_TYPE_TO_STATUS mapping in _streaming.py
• Added _get_status_code_from_sse_error() helper function
• Modified error handling in both Stream and AsyncStream to use the mapped status code

Testing

This fix ensures that:

• overloaded_error → OverloadedError(status_code=529)
• rate_limit_error → RateLimitError(status_code=429)
• api_error → InternalServerError(status_code=500)
• etc.

Fixes: #1258