fix(proxy): clean up model override env vars when switching providers in takeover mode

When proxy takeover is enabled, switching providers no longer writes to
the Live config. However, if model override fields (ANTHROPIC_MODEL,
ANTHROPIC_REASONING_MODEL, etc.) remain in the Live config, Claude Code
continues sending requests with the old model name, causing failures
when the new provider doesn't support that model.

This fix:
- Removes model override env keys from Claude Live config during takeover
- Adds cleanup when switching providers in takeover mode
- Fixes has_mapping() to include reasoning_model in the check
- Adds test coverage for reasoning-only model mapping scenarios

src-tauri/src/proxy/model_mapper.rs

@@ -54,6 +54,7 @@ impl ModelMapping {
             || self.sonnet_model.is_some()
             || self.opus_model.is_some()
             || self.default_model.is_some()
+            || self.reasoning_model.is_some()
     }
 
     /// 根据原始模型名称获取映射后的模型
@@ -182,6 +183,27 @@ mod tests {
         }
     }
 
+    fn create_provider_with_reasoning_only() -> Provider {
+        Provider {
+            id: "test".to_string(),
+            name: "Test".to_string(),
+            settings_config: json!({
+                "env": {
+                    "ANTHROPIC_REASONING_MODEL": "reasoning-only-model"
+                }
+            }),
+            website_url: None,
+            category: None,
+            created_at: None,
+            sort_index: None,
+            notes: None,
+            meta: None,
+            icon: None,
+            icon_color: None,
+            in_failover_queue: false,
+        }
+    }
+
     #[test]
     fn test_sonnet_mapping() {
         let provider = create_provider_with_mapping();
@@ -222,6 +244,31 @@ mod tests {
         assert_eq!(mapped, Some("reasoning-model".to_string()));
     }
 
+    #[test]
+    fn test_reasoning_only_mapping_in_thinking_mode() {
+        let provider = create_provider_with_reasoning_only();
+        let body = json!({
+            "model": "claude-sonnet-4-5",
+            "thinking": {"type": "enabled"}
+        });
+        let (result, _, mapped) = apply_model_mapping(body, &provider);
+        assert_eq!(result["model"], "reasoning-only-model");
+        assert_eq!(mapped, Some("reasoning-only-model".to_string()));
+    }
+
+    #[test]
+    fn test_reasoning_only_mapping_does_not_affect_non_thinking() {
+        let provider = create_provider_with_reasoning_only();
+        let body = json!({
+            "model": "claude-sonnet-4-5",
+            "thinking": {"type": "disabled"}
+        });
+        let (result, original, mapped) = apply_model_mapping(body, &provider);
+        assert_eq!(result["model"], "claude-sonnet-4-5");
+        assert_eq!(original, Some("claude-sonnet-4-5".to_string()));
+        assert!(mapped.is_none());
+    }
+
     #[test]
     fn test_thinking_disabled() {
         let provider = create_provider_with_mapping();