Remove stray .snap.new filtering from schema fixtures reader

Motivation:
- `codex-app-server-protocol` schema fixtures are not insta snapshots.
- Filtering `*.snap.new` is confusing and risks hiding real files.

What changed:
- Remove the `.snap.new` filename special case in `read_schema_fixture_tree`.

Refs: macOS Bazel schema fixture mismatch investigation

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

@@ -0,0 +1,48 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "definitions": {
+    "JSONRPCErrorError": {
+      "properties": {
+        "code": {
+          "format": "int64",
+          "type": "integer"
+        },
+        "data": true,
+        "message": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "code",
+        "message"
+      ],
+      "type": "object"
+    },
+    "RequestId": {
+      "anyOf": [
+        {
+          "type": "string"
+        },
+        {
+          "format": "int64",
+          "type": "integer"
+        }
+      ]
+    }
+  },
+  "description": "A response to a request that indicates an error occurred.",
+  "properties": {
+    "error": {
+      "$ref": "#/definitions/JSONRPCErrorError"
+    },
+    "id": {
+      "$ref": "#/definitions/RequestId"
+    }
+  },
+  "required": [
+    "error",
+    "id"
+  ],
+  "title": "JSONRPCError",
+  "type": "object"
+}