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/v2/ContextCompactedNotification.json

@@ -0,0 +1,18 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "description": "Deprecated: Use `ContextCompaction` item type instead.",
+  "properties": {
+    "threadId": {
+      "type": "string"
+    },
+    "turnId": {
+      "type": "string"
+    }
+  },
+  "required": [
+    "threadId",
+    "turnId"
+  ],
+  "title": "ContextCompactedNotification",
+  "type": "object"
+}