Fix streaming request failover to retry on error status codes

[GoJam11]

Summary

This PR fixes the failover behavior for streaming requests to properly retry and fall back when the upstream returns error status codes before any response body is streamed to the client.

Key Changes

• Removed incorrect streaming request check: Deleted the !retryableStreamRequest condition that was preventing status-based failover for streaming requests. Error status codes are returned before the response body is streamed, making retries safe at that point.
• Updated test expectations:
  • Changed test "does NOT retry a streaming request on 5xx" to "retries a streaming request when the upstream returns an error status before the first byte" to reflect the corrected behavior
  • Added new test "falls over a streaming request to the next provider on 5xx" to verify model fallback works for streaming requests
• Added clarifying comments: Documented that error status codes arrive before streaming begins, so failover is safe; only after streaming has started (with 2xx status) should retries be skipped

Implementation Details

The key insight is that when an upstream returns an error status code (4xx, 5xx), the response headers and status are sent before any body content. For streaming requests, this means the error status arrives before the first byte of the stream is sent to the client, so the response can be safely cancelled and retried/fallen back to another provider. Only after a successful (2xx) response has started streaming should retries be disabled, but those cases won't trigger failover anyway since they have success status codes.

https://claude.ai/code/session_01B1f8CF7YQmp9EU3m8yJYFr