fix(FN-2688): add root dir to plugin install mock task store

- Add getRootDir() to the plugin command mock TaskStore used by createPluginLoader
- Ensure fn plugin install can resolve project root expectations without runtime failures
- Add a patch changeset for @runfusion/fusion documenting the CLI fix

.changeset/fn-2688-plugin-install-mock-taskstore.md

@@ -0,0 +1,5 @@
+---
+"@runfusion/fusion": patch
+---
+
+Fix `fn plugin install` failing in CLI plugin commands by adding `getRootDir()` to the mock TaskStore used by `createPluginLoader`.