[codex] Reuse Apps MCP path override for plugin-service rollout (#22527)

## Summary
- reuse `apps_mcp_path_override` for the plugin-service rollout,
defaulting enabled boolean overrides to `/ps/mcp` while preserving
explicit configured paths

## Validation
- `just write-config-schema`
- `just fmt`
- `cargo test -p codex-mcp`
- `cargo test -p codex-core apps_mcp_path_override`
- `cargo test -p codex-core
to_mcp_config_preserves_apps_feature_from_config`
- `cargo test -p codex-features`

codex-rs/core/src/config/config_tests.rs

@@ -8769,6 +8769,58 @@ path = "/custom/mcp"
     Ok(())
 }
 
+#[tokio::test]
+async fn config_defaults_enabled_apps_mcp_path_override_to_plugin_service() -> std::io::Result<()> {
+    let codex_home = TempDir::new()?;
+    let toml = r#"
+model = "gpt-5.4"
+
+[features]
+apps_mcp_path_override = true
+"#;
+    let cfg: ConfigToml =
+        toml::from_str(toml).expect("TOML deserialization should succeed for apps MCP feature");
+
+    let config = Config::load_from_base_config_with_overrides(
+        cfg,
+        ConfigOverrides::default(),
+        codex_home.abs(),
+    )
+    .await?;
+
+    assert!(config.features.enabled(Feature::AppsMcpPathOverride));
+    assert_eq!(config.apps_mcp_path_override.as_deref(), Some("/ps/mcp"));
+    Ok(())
+}
+
+#[tokio::test]
+async fn config_preserves_explicit_apps_mcp_path_override_path() -> std::io::Result<()> {
+    let codex_home = TempDir::new()?;
+    let toml = r#"
+model = "gpt-5.4"
+
+[features.apps_mcp_path_override]
+enabled = true
+path = "/custom/mcp"
+"#;
+    let cfg: ConfigToml =
+        toml::from_str(toml).expect("TOML deserialization should succeed for apps MCP feature");
+
+    let config = Config::load_from_base_config_with_overrides(
+        cfg,
+        ConfigOverrides::default(),
+        codex_home.abs(),
+    )
+    .await?;
+
+    assert_eq!(
+        config.apps_mcp_path_override.as_deref(),
+        Some("/custom/mcp")
+    );
+    assert!(config.features.enabled(Feature::AppsMcpPathOverride));
+    Ok(())
+}
+
 #[tokio::test]
 async fn config_loads_mcp_oauth_callback_url_from_toml() -> std::io::Result<()> {
     let codex_home = TempDir::new()?;

codex-rs/core/src/config/mod.rs

@@ -2615,6 +2615,7 @@ impl Config {
                 .and_then(|config| config.path.as_ref())
                 .or_else(|| base.and_then(|config| config.path.as_ref()))
                 .cloned()
+                .or_else(|| Some("/ps/mcp".to_string()))
         } else {
             None
         };