## What changed
Recognize `access_program_not_enabled` in HTTP 403 responses and response stream errors as a dedicated, non-retryable error. Preserve HTTP diagnostic details and provider stream messages, with a default message when none is supplied.
Stop SSE streams immediately on this error and avoid triggering WebSocket fallback for explicit access program denials. Add a fixed diagnostic label for doctor output and telemetry.
## Testing
Extend coverage for HTTP and WebSocket denials, immediate SSE termination despite retry headers, and doctor diagnostic redaction.
GitOrigin-RevId: 3a41f7bb80ad5e13a87e831b458d17cdd19a4b62