fix: strip -plugin suffix in deriveIdHint to match manifest ids (#85170)

The deriveIdHint function already strips -provider from unscoped
package names (@openclaw/anthropic-provider -> anthropic) but does
not strip -plugin (@openclaw/xai-plugin -> xai-plugin instead of
xai). This causes ~30 spurious 'plugin id mismatch' warnings on
gateway startup for built-in plugins whose package names end in
-plugin.

Closes #85048

src/plugins/discovery.ts

@@ -550,7 +550,9 @@ function deriveIdHint(params: {
   const normalizedPackageId =
     unscoped.endsWith("-provider") && unscoped.length > "-provider".length
       ? unscoped.slice(0, -"-provider".length)
-      : unscoped;
+      : unscoped.endsWith("-plugin") && unscoped.length > "-plugin".length
+        ? unscoped.slice(0, -"-plugin".length)
+        : unscoped;
 
   if (!params.hasMultipleExtensions) {
     return normalizedPackageId;