make model optional in config (#7769)

- Make Config.model optional and centralize default-selection logic in
ModelsManager, including a default_model helper (with
codex-auto-balanced when available) so sessions now carry an explicit
chosen model separate from the base config.
- Resolve `model` once in `core` and `tui` from config. Then store the
state of it on other structs.
- Move refreshing models to be before resolving the default model

codex-rs/core/tests/common/responses.rs

@@ -689,6 +689,33 @@ pub async fn start_mock_server() -> MockServer {
     server
 }
 
+// todo(aibrahim): remove this and use our search matching patterns directly
+/// Get all POST requests to `/responses` endpoints from the mock server.
+/// Filters out GET requests (e.g., `/models`) .
+pub async fn get_responses_requests(server: &MockServer) -> Vec<wiremock::Request> {
+    server
+        .received_requests()
+        .await
+        .expect("mock server should not fail")
+        .into_iter()
+        .filter(|req| req.method == "POST" && req.url.path().ends_with("/responses"))
+        .collect()
+}
+
+// todo(aibrahim): remove this and use our search matching patterns directly
+/// Get request bodies as JSON values from POST requests to `/responses` endpoints.
+/// Filters out GET requests (e.g., `/models`) .
+pub async fn get_responses_request_bodies(server: &MockServer) -> Vec<Value> {
+    get_responses_requests(server)
+        .await
+        .into_iter()
+        .map(|req| {
+            req.body_json::<Value>()
+                .expect("request body to be valid JSON")
+        })
+        .collect()
+}
+
 #[derive(Clone)]
 pub struct FunctionCallResponseMocks {
     pub function_call: ResponseMock,
@@ -769,6 +796,10 @@ pub async fn mount_sse_sequence(server: &MockServer, bodies: Vec<String>) -> Res
 /// - Additionally, enforce symmetry: every `function_call`/`custom_tool_call`
 ///   in the `input` must have a matching output entry.
 fn validate_request_body_invariants(request: &wiremock::Request) {
+    // Skip GET requests (e.g., /models)
+    if request.method != "POST" || !request.url.path().ends_with("/responses") {
+        return;
+    }
     let Ok(body): Result<Value, _> = request.body_json() else {
         return;
     };