Add a getUserAgent MCP method (#3320)

This will allow the extension to pass this user agent + a suffix for its
requests

codex-rs/protocol/src/mcp_protocol.rs

@@ -137,6 +137,10 @@ pub enum ClientRequest {
         #[serde(rename = "id")]
         request_id: RequestId,
     },
+    GetUserAgent {
+        #[serde(rename = "id")]
+        request_id: RequestId,
+    },
     /// Execute a command (argv vector) under the server's sandbox.
     ExecOneOffCommand {
         #[serde(rename = "id")]
@@ -339,6 +343,12 @@ pub struct GetAuthStatusResponse {
     pub auth_token: Option<String>,
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, TS)]
+#[serde(rename_all = "camelCase")]
+pub struct GetUserAgentResponse {
+    pub user_agent: String,
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, TS)]
 #[serde(rename_all = "camelCase")]
 pub struct GetUserSavedConfigResponse {