Handle new refusal stop reason from Claude 4 models (#31217)

This PR adds support for handling the new [`refusal` stop
reason](https://docs.anthropic.com/en/docs/test-and-evaluate/strengthen-guardrails/handle-streaming-refusals)
from Claude 4 models.

<img width="409" alt="Screenshot 2025-05-22 at 4 31 56 PM"
src="https://github.com/user-attachments/assets/707b04f5-5a52-4a19-95d9-cbd2be2dd86f"
/>

Release Notes:

- Added handling for `"stop_reason": "refusal"` from Claude 4 models.

crates/eval/src/example.rs

@@ -231,6 +231,10 @@ impl ExampleContext {
                     Ok(StopReason::MaxTokens) => {
                         tx.try_send(Err(anyhow!("Exceeded maximum tokens"))).ok();
                     }
+                    Ok(StopReason::Refusal) => {
+                        tx.try_send(Err(anyhow!("Model refused to generate content")))
+                            .ok();
+                    }
                     Err(err) => {
                         tx.try_send(Err(anyhow!(err.clone()))).ok();
                     }