refactor(proxy): remove Codex Chat Completions API format support

Codex CLI only uses OpenAI Responses API. Remove unused Chat Completions
routes, handler, parser config, URL builder branches, frontend API format
selector, and stale i18n keys. Claude openai_chat support is preserved.

src-tauri/src/services/proxy.rs

@@ -814,7 +814,7 @@ impl ProxyService {
     ///
     /// 代理服务器的路由已经根据 API 端点自动区分应用类型：
     /// - `/v1/messages` → Claude
-    /// - `/v1/chat/completions`, `/v1/responses` → Codex
+    /// - `/v1/responses` → Codex
     /// - `/v1beta/*` → Gemini
     ///
     /// 因此不需要在 URL 中添加应用前缀。