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/ClientRequest.json

@@ -1131,6 +1131,17 @@
       },
       "type": "object"
     },
+    "PluginUninstallParams": {
+      "properties": {
+        "pluginId": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "pluginId"
+      ],
+      "type": "object"
+    },
     "ProductSurface": {
       "enum": [
         "chatgpt",
@@ -3571,6 +3582,30 @@
       "title": "Plugin/installRequest",
       "type": "object"
     },
+    {
+      "properties": {
+        "id": {
+          "$ref": "#/definitions/RequestId"
+        },
+        "method": {
+          "enum": [
+            "plugin/uninstall"
+          ],
+          "title": "Plugin/uninstallRequestMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/PluginUninstallParams"
+        }
+      },
+      "required": [
+        "id",
+        "method",
+        "params"
+      ],
+      "title": "Plugin/uninstallRequest",
+      "type": "object"
+    },
     {
       "properties": {
         "id": {