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/assistant_context_editor/src/context.rs

@@ -2204,6 +2204,7 @@ impl AssistantContext {
                             StopReason::ToolUse => {}
                             StopReason::EndTurn => {}
                             StopReason::MaxTokens => {}
+                            StopReason::Refusal => {}
                         }
                     }
                 })