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/services/provider/mod.rs

@@ -292,6 +292,14 @@ impl ProviderService {
             )
             .map_err(|e| AppError::Message(format!("更新 Live 备份失败: {e}")))?;
 
+            // 关键修复：接管模式下切换供应商不会写回 Live 配置，
+            // 需要主动清理 Claude Live 中的“模型覆盖”字段，避免仍以旧模型名发起请求。
+            if matches!(app_type, AppType::Claude) {
+                if let Err(e) = state.proxy_service.cleanup_claude_model_overrides_in_live() {
+                    log::warn!("清理 Claude Live 模型字段失败（不影响切换结果）: {e}");
+                }
+            }
+
             // Note: No Live config write, no MCP sync
             // The proxy server will route requests to the new provider via is_current
             return Ok(());