feat(codex-oauth): fetch model list from ChatGPT backend on demand

- Add `get_codex_oauth_models` Tauri command reusing the managed OAuth
  access token to hit `chatgpt.com/backend-api/codex/models`; HTTP and
  multi-shape JSON parsing live in `services::codex_oauth_models` so the
  command stays thin.
- Unify the Claude form's "fetch models" button across normal / Copilot /
  Codex OAuth presets, drop the auto-load effect for Copilot in favor of
  explicit clicks, and guard against stale responses with a requestId ref.
- Add Vitest coverage for both Copilot and Codex OAuth paths asserting no
  request on mount and the correct account id on click; add Rust unit
  tests for the four model-list payload shapes.

src/lib/api/model-fetch.ts

@@ -27,6 +27,19 @@ export async function fetchModelsForConfig(
   });
 }
 
+/**
+ * 获取 Codex OAuth (ChatGPT Plus/Pro 反代) 可用模型列表
+ *
+ * Codex OAuth 使用 ChatGPT 的 backend-api/codex 端点，不兼容普通 /v1/models。
+ */
+export async function fetchCodexOauthModels(
+  accountId?: string | null,
+): Promise<FetchedModel[]> {
+  return invoke("get_codex_oauth_models", {
+    accountId: accountId || null,
+  });
+}
+
 /**
  * 根据错误类型显示对应的 toast 提示
  */