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-tauri/src/lib.rs

@@ -1105,6 +1105,7 @@ pub fn run() {
             // subscription quota
             commands::get_subscription_quota,
             commands::get_codex_oauth_quota,
+            commands::get_codex_oauth_models,
             commands::get_coding_plan_quota,
             commands::get_balance,
             // New MCP via config.json (SSOT)