Serialize plugin install requests (#32894)

## What changed

- Mark `request_plugin_install` as not supporting parallel tool calls so install requests execute serially.
- Add a unit test covering the handler's parallel-call capability.

GitOrigin-RevId: 5f2c8184562bd776b0e527e5e258c61c604a0910

codex-rs/core/src/tools/handlers/request_plugin_install.rs

@@ -79,7 +79,7 @@ impl ToolExecutor<ToolInvocation> for RequestPluginInstallHandler {
     }
 
     fn supports_parallel_tool_calls(&self) -> bool {
-        true
+        false
     }
 
     fn handle(&self, invocation: ToolInvocation) -> codex_tools::ToolExecutorFuture<'_> {

codex-rs/core/src/tools/handlers/request_plugin_install_tests.rs

@@ -21,6 +21,16 @@ use rmcp::model::ElicitationAction;
 use serde_json::json;
 use tempfile::tempdir;
 
+#[test]
+fn request_plugin_install_does_not_support_parallel_tool_calls() {
+    let handler = RequestPluginInstallHandler::new(
+        Vec::new(),
+        ToolSuggestPresentation::RecommendationContext,
+    );
+
+    assert!(!handler.supports_parallel_tool_calls());
+}
+
 #[tokio::test]
 async fn verified_plugin_install_completed_requires_installed_plugin() {
     let codex_home = tempdir().expect("tempdir should succeed");