feat: added SetDefaultModel to JSON-RPC server (#3512)

This adds `SetDefaultModel`, which takes `model` and `reasoning_effort`
as optional fields. If set, the field will overwrite what is in the
user's `config.toml`.

This reuses logic that was added to support the `/model` command in the
TUI: https://github.com/openai/codex/pull/2799.

codex-rs/mcp-server/tests/suite/set_default_model.rs

@@ -0,0 +1,62 @@
+use codex_core::config::ConfigToml;
+use codex_protocol::config_types::ReasoningEffort;
+use codex_protocol::mcp_protocol::SetDefaultModelParams;
+use codex_protocol::mcp_protocol::SetDefaultModelResponse;
+use mcp_test_support::McpProcess;
+use mcp_test_support::to_response;
+use mcp_types::JSONRPCResponse;
+use mcp_types::RequestId;
+use pretty_assertions::assert_eq;
+use tempfile::TempDir;
+use tokio::time::timeout;
+
+const DEFAULT_READ_TIMEOUT: std::time::Duration = std::time::Duration::from_secs(10);
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn set_default_model_persists_overrides() {
+    let codex_home = TempDir::new().unwrap_or_else(|e| panic!("create tempdir: {e}"));
+
+    let mut mcp = McpProcess::new(codex_home.path())
+        .await
+        .expect("spawn mcp process");
+    timeout(DEFAULT_READ_TIMEOUT, mcp.initialize())
+        .await
+        .expect("init timeout")
+        .expect("init failed");
+
+    let params = SetDefaultModelParams {
+        model: Some("o4-mini".to_string()),
+        reasoning_effort: Some(ReasoningEffort::High),
+    };
+
+    let request_id = mcp
+        .send_set_default_model_request(params)
+        .await
+        .expect("send setDefaultModel");
+
+    let resp: JSONRPCResponse = timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_response_message(RequestId::Integer(request_id)),
+    )
+    .await
+    .expect("setDefaultModel timeout")
+    .expect("setDefaultModel response");
+
+    let _: SetDefaultModelResponse =
+        to_response(resp).expect("deserialize setDefaultModel response");
+
+    let config_path = codex_home.path().join("config.toml");
+    let config_contents = tokio::fs::read_to_string(&config_path)
+        .await
+        .expect("read config.toml");
+    let config_toml: ConfigToml = toml::from_str(&config_contents).expect("parse config.toml");
+
+    assert_eq!(
+        ConfigToml {
+            model: Some("o4-mini".to_string()),
+            model_reasoning_effort: Some(ReasoningEffort::High),
+            ..Default::default()
+        },
+        config_toml,
+    );
+}