fix: map adaptive thinking to xhigh reasoning_effort instead of high

When thinking.type is "adaptive" (Claude's maximum thinking mode) and
output_config.effort is absent, resolve_reasoning_effort() incorrectly
mapped it to "high" instead of "xhigh" in OpenAI format conversions.

src-tauri/src/proxy/providers/transform_responses.rs

@@ -1047,7 +1047,7 @@ mod tests {
     }
 
     #[test]
-    fn test_responses_thinking_adaptive_sets_reasoning_high() {
+    fn test_responses_thinking_adaptive_sets_reasoning_xhigh() {
         let input = json!({
             "model": "gpt-5.4",
             "max_tokens": 1024,
@@ -1056,7 +1056,7 @@ mod tests {
         });
 
         let result = anthropic_to_responses(input, None, false).unwrap();
-        assert_eq!(result["reasoning"]["effort"], "high");
+        assert_eq!(result["reasoning"]["effort"], "xhigh");
     }
 
     #[test]