feat(app-server): gate permission confirmations by capability

Add initialize-time server request capabilities so app-server only sends conversational permission confirmation requests to clients that advertise support.

Unsupported clients still fail closed without changing permissions, while capable clients receive the existing request/response flow for narrow grants and preset picker requests.

codex-rs/app-server-protocol/schema/json/v1/InitializeParams.json

@@ -39,9 +39,27 @@
             "array",
             "null"
           ]
+        },
+        "supportedServerRequests": {
+          "description": "Server request methods this connection knows how to render and answer.",
+          "items": {
+            "$ref": "#/definitions/SupportedServerRequestMethod"
+          },
+          "type": [
+            "array",
+            "null"
+          ]
         }
       },
       "type": "object"
+    },
+    "SupportedServerRequestMethod": {
+      "description": "Server-initiated request methods a client can opt into during initialize.",
+      "enum": [
+        "item/permissions/requestApproval",
+        "item/permissionPreset/requestApproval"
+      ],
+      "type": "string"
     }
   },
   "properties": {