refactor(proxy): switch OpenRouter to passthrough mode for native Claude API

OpenRouter now supports Claude Code compatible endpoint (/v1/messages),
eliminating the need for Anthropic ↔ OpenAI format conversion.

- Disable format transformation for OpenRouter (keep old logic as fallback)
- Pass through original endpoint instead of redirecting to /v1/chat/completions
- Add anthropic-version header for ClaudeAuth and Bearer strategies
- Update tests to reflect new passthrough behavior

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

@@ -87,7 +87,7 @@ pub trait ProviderAdapter: Send + Sync {
     /// 是否需要格式转换
     ///
     /// 默认返回 `false`（透传模式）。
-    /// 仅当供应商需要格式转换时（如 Claude + OpenRouter）才返回 `true`。
+    /// 仅当供应商需要格式转换时（如 Claude + OpenRouter 旧 OpenAI 兼容接口）才返回 `true`。
     ///
     /// # Arguments
     /// * `provider` - Provider 配置