[app-server] Add a method to override feature flags. (#15601)

- [x] Add a method to override feature flags globally and not just
thread level.

codex-rs/app-server-protocol/schema/json/ClientRequest.json

@@ -524,6 +524,21 @@
       ],
       "type": "object"
     },
+    "ExperimentalFeatureEnablementSetParams": {
+      "properties": {
+        "enablement": {
+          "additionalProperties": {
+            "type": "boolean"
+          },
+          "description": "Process-wide runtime feature enablement keyed by canonical feature name.\n\nOnly named features are updated. Omitted features are left unchanged. Send an empty map for a no-op.",
+          "type": "object"
+        }
+      },
+      "required": [
+        "enablement"
+      ],
+      "type": "object"
+    },
     "ExperimentalFeatureListParams": {
       "properties": {
         "cursor": {
@@ -4294,6 +4309,30 @@
       "title": "ExperimentalFeature/listRequest",
       "type": "object"
     },
+    {
+      "properties": {
+        "id": {
+          "$ref": "#/definitions/RequestId"
+        },
+        "method": {
+          "enum": [
+            "experimentalFeature/enablement/set"
+          ],
+          "title": "ExperimentalFeature/enablement/setRequestMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/ExperimentalFeatureEnablementSetParams"
+        }
+      },
+      "required": [
+        "id",
+        "method",
+        "params"
+      ],
+      "title": "ExperimentalFeature/enablement/setRequest",
+      "type": "object"
+    },
     {
       "properties": {
         "id": {