Add models endpoint (#7603)

- Use the codex-api crate to introduce models endpoint. 
- Add `models` to codex core tests helpers
- Add `ModelsInfo` for the endpoint return type

codex-rs/codex-api/tests/models_integration.rs

@@ -0,0 +1,100 @@
+use codex_api::AuthProvider;
+use codex_api::ModelsClient;
+use codex_api::provider::Provider;
+use codex_api::provider::RetryConfig;
+use codex_api::provider::WireApi;
+use codex_client::ReqwestTransport;
+use codex_protocol::openai_models::ClientVersion;
+use codex_protocol::openai_models::ModelInfo;
+use codex_protocol::openai_models::ModelVisibility;
+use codex_protocol::openai_models::ModelsResponse;
+use codex_protocol::openai_models::ReasoningLevel;
+use codex_protocol::openai_models::ShellType;
+use http::HeaderMap;
+use http::Method;
+use wiremock::Mock;
+use wiremock::MockServer;
+use wiremock::ResponseTemplate;
+use wiremock::matchers::method;
+use wiremock::matchers::path;
+
+#[derive(Clone, Default)]
+struct DummyAuth;
+
+impl AuthProvider for DummyAuth {
+    fn bearer_token(&self) -> Option<String> {
+        None
+    }
+}
+
+fn provider(base_url: &str) -> Provider {
+    Provider {
+        name: "test".to_string(),
+        base_url: base_url.to_string(),
+        query_params: None,
+        wire: WireApi::Responses,
+        headers: HeaderMap::new(),
+        retry: RetryConfig {
+            max_attempts: 1,
+            base_delay: std::time::Duration::from_millis(1),
+            retry_429: false,
+            retry_5xx: true,
+            retry_transport: true,
+        },
+        stream_idle_timeout: std::time::Duration::from_secs(1),
+    }
+}
+
+#[tokio::test]
+async fn models_client_hits_models_endpoint() {
+    let server = MockServer::start().await;
+    let base_url = format!("{}/api/codex", server.uri());
+
+    let response = ModelsResponse {
+        models: vec![ModelInfo {
+            slug: "gpt-test".to_string(),
+            display_name: "gpt-test".to_string(),
+            description: Some("desc".to_string()),
+            default_reasoning_level: ReasoningLevel::Medium,
+            supported_reasoning_levels: vec![
+                ReasoningLevel::Low,
+                ReasoningLevel::Medium,
+                ReasoningLevel::High,
+            ],
+            shell_type: ShellType::ShellCommand,
+            visibility: ModelVisibility::List,
+            minimal_client_version: ClientVersion(0, 1, 0),
+            supported_in_api: true,
+            priority: 1,
+        }],
+    };
+
+    Mock::given(method("GET"))
+        .and(path("/api/codex/models"))
+        .respond_with(
+            ResponseTemplate::new(200)
+                .insert_header("content-type", "application/json")
+                .set_body_json(&response),
+        )
+        .mount(&server)
+        .await;
+
+    let transport = ReqwestTransport::new(reqwest::Client::new());
+    let client = ModelsClient::new(transport, provider(&base_url), DummyAuth);
+
+    let result = client
+        .list_models("0.1.0", HeaderMap::new())
+        .await
+        .expect("models request should succeed");
+
+    assert_eq!(result.models.len(), 1);
+    assert_eq!(result.models[0].slug, "gpt-test");
+
+    let received = server
+        .received_requests()
+        .await
+        .expect("should capture requests");
+    assert_eq!(received.len(), 1);
+    assert_eq!(received[0].method, Method::GET.as_str());
+    assert_eq!(received[0].url.path(), "/api/codex/models");
+}