chore: plugin/uninstall endpoint (#14111)

add `plugin/uninstall` app-server endpoint to fully rm plugin from
plugins cache dir and rm entry from user config file.

plugin-enablement is session-scoped, so uninstalls are only picked up in
new sessions (like installs).

added tests.

codex-rs/app-server-protocol/schema/json/v2/PluginUninstallResponse.json

@@ -0,0 +1,5 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "title": "PluginUninstallResponse",
+  "type": "object"
+}