Fix clippy in app-server schema fixture test

Motivation:
- `cargo clippy --tests` is run in CI with `-D clippy::expect-used`; the new schema fixture test used `expect()`.

What changed:
- Convert test helpers to return `anyhow::Result` and add context instead of using `expect()`.

Refs: 3b069fda39 (schema fixture vendoring commit)

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

@@ -0,0 +1,22 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "oneOf": [
+    {
+      "properties": {
+        "method": {
+          "enum": [
+            "initialized"
+          ],
+          "title": "InitializedNotificationMethod",
+          "type": "string"
+        }
+      },
+      "required": [
+        "method"
+      ],
+      "title": "InitializedNotification",
+      "type": "object"
+    }
+  ],
+  "title": "ClientNotification"
+}