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/language_models/src/provider/anthropic.rs

@@ -825,6 +825,7 @@ impl AnthropicEventMapper {
                         "end_turn" => StopReason::EndTurn,
                         "max_tokens" => StopReason::MaxTokens,
                         "tool_use" => StopReason::ToolUse,
+                        "refusal" => StopReason::Refusal,
                         _ => {
                             log::error!("Unexpected anthropic stop_reason: {stop_reason}");
                             StopReason::EndTurn