Strengthen Agent Plugin MCP environment overlay tests (#40410)

## What changed

- Cover forwarding a legacy `env_vars` entry when the portable MCP server has no `env` map.
- Use the test workspace variable instead of `PATH` in the stdio round-trip test, keeping overlay forwarding distinct from portable environment expansion.

GitOrigin-RevId: c5cfb333e553581fc856d43a04e5be0abd9f65ef

codex-rs/core-plugins/src/loader_tests.rs

@@ -116,7 +116,7 @@ async fn agent_plugin_codex_mcp_overlay_only_forwards_matching_stdio_server_env_
 }
 
 #[tokio::test]
-async fn agent_plugin_codex_mcp_overlay_supports_inline_legacy_servers() {
+async fn agent_plugin_codex_mcp_overlay_supports_inline_legacy_servers_without_portable_env() {
     let temp_dir = TempDir::new().expect("tempdir");
     let plugin_root = temp_dir.path().join("plugin");
     write_file(
@@ -130,8 +130,7 @@ async fn agent_plugin_codex_mcp_overlay_supports_inline_legacy_servers() {
   "mcpServers": {
     "shared": {
       "type": "stdio",
-      "command": "portable-server",
-      "env": {"TOKEN": "${TOKEN}"}
+      "command": "portable-server"
     }
   }
 }"#,
@@ -139,13 +138,12 @@ async fn agent_plugin_codex_mcp_overlay_supports_inline_legacy_servers() {
 
     let mut expected = load_plugin_mcp_servers(&plugin_root, /*auth_mode*/ None).await;
     let Some(McpServerConfig {
-        transport: McpServerTransportConfig::Stdio { env, env_vars, .. },
+        transport: McpServerTransportConfig::Stdio { env_vars, .. },
         ..
     }) = expected.get_mut("shared")
     else {
         panic!("expected portable stdio server");
     };
-    env.as_mut().expect("portable environment").remove("TOKEN");
     env_vars.push("TOKEN".into());
 
     write_file(