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/codex_app_server_protocol.schemas.json

@@ -860,6 +860,30 @@
           "title": "Plugin/installRequest",
           "type": "object"
         },
+        {
+          "properties": {
+            "id": {
+              "$ref": "#/definitions/v2/RequestId"
+            },
+            "method": {
+              "enum": [
+                "plugin/uninstall"
+              ],
+              "title": "Plugin/uninstallRequestMethod",
+              "type": "string"
+            },
+            "params": {
+              "$ref": "#/definitions/v2/PluginUninstallParams"
+            }
+          },
+          "required": [
+            "id",
+            "method",
+            "params"
+          ],
+          "title": "Plugin/uninstallRequest",
+          "type": "object"
+        },
         {
           "properties": {
             "id": {
@@ -12565,6 +12589,24 @@
         ],
         "type": "object"
       },
+      "PluginUninstallParams": {
+        "$schema": "http://json-schema.org/draft-07/schema#",
+        "properties": {
+          "pluginId": {
+            "type": "string"
+          }
+        },
+        "required": [
+          "pluginId"
+        ],
+        "title": "PluginUninstallParams",
+        "type": "object"
+      },
+      "PluginUninstallResponse": {
+        "$schema": "http://json-schema.org/draft-07/schema#",
+        "title": "PluginUninstallResponse",
+        "type": "object"
+      },
       "ProductSurface": {
         "enum": [
           "chatgpt",