fix(fixtures): add $schema to fixtures schema

Author: S1M0N38

tests/fixtures/fixtures.schema.json

@@ -2,6 +2,12 @@
   "$schema": "http://json-schema.org/draft-07/schema#",
   "title": "Balatro API Test Fixtures Schema",
   "type": "object",
+  "properties": {
+    "$schema": {
+      "type": "string",
+      "description": "JSON Schema reference"
+    }
+  },
   "additionalProperties": {
     "type": "object",
     "additionalProperties": {