chore: update .gitignore for Yarn Modern and untrack install-state.gz

Author: gspencergoog

.gitignore

@@ -25,3 +25,12 @@ a2a_agents/python/a2ui_agent/src/a2ui/assets/**/*.json
 agent_sdks/python/src/a2ui/assets/**/*.json
 ## Generated files for sandbox frame.
 samples/client/angular/projects/mcp_calculator/public/mcp_apps_inner_iframe/
+
+# Yarn Modern
+.yarn/*
+!.yarn/patches
+!.yarn/plugins
+!.yarn/releases
+!.yarn/sdks
+!.yarn/versions
+.yarn/cache