feat: support list mcp servers in app server (#7505)

### Summary
Added `mcp/servers/list` which is equivalent to `/mcp` slash command in
CLI for response. This will be used in VSCE MCP settings to show log in
status, available tools etc.

codex-rs/app-server-protocol/src/protocol/common.rs

@@ -139,6 +139,11 @@ client_request_definitions! {
         response: v2::ModelListResponse,
     },
 
+    McpServersList => "mcpServers/list" {
+        params: v2::ListMcpServersParams,
+        response: v2::ListMcpServersResponse,
+    },
+
     LoginAccount => "account/login/start" {
         params: v2::LoginAccountParams,
         response: v2::LoginAccountResponse,