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/agent/src/agent_diff.rs

@@ -1348,6 +1348,7 @@ impl AgentDiff {
             ThreadEvent::NewRequest
             | ThreadEvent::Stopped(Ok(StopReason::EndTurn))
             | ThreadEvent::Stopped(Ok(StopReason::MaxTokens))
+            | ThreadEvent::Stopped(Ok(StopReason::Refusal))
             | ThreadEvent::Stopped(Err(_))
             | ThreadEvent::ShowError(_)
             | ThreadEvent::CompletionCanceled => {