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/protocol/src/mcp_protocol.rs

@@ -153,6 +153,11 @@ pub enum ClientRequest {
         #[serde(rename = "id")]
         request_id: RequestId,
     },
+    SetDefaultModel {
+        #[serde(rename = "id")]
+        request_id: RequestId,
+        params: SetDefaultModelParams,
+    },
     GetUserAgent {
         #[serde(rename = "id")]
         request_id: RequestId,
@@ -416,6 +421,19 @@ pub struct GetUserSavedConfigResponse {
     pub config: UserSavedConfig,
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, TS)]
+#[serde(rename_all = "camelCase")]
+pub struct SetDefaultModelParams {
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub model: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub reasoning_effort: Option<ReasoningEffort>,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, TS)]
+#[serde(rename_all = "camelCase")]
+pub struct SetDefaultModelResponse {}
+
 /// UserSavedConfig contains a subset of the config. It is meant to expose mcp
 /// client-configurable settings that can be specified in the NewConversation
 /// and SendUserTurn requests.