Add negative tests for new encoding fields

Author: handrews

tests/schema/fail/media-type-enc-item-exclusion.yaml

@@ -0,0 +1,10 @@
+openapi: 3.2.0
+info:
+  title: API
+  version: 1.0.0
+components:
+  requestBodies:
+    content:
+      multipart/mixed:
+        encoding: {}
+        itemEncoding: {}

tests/schema/fail/media-type-enc-prefix-exclusion.yaml

@@ -0,0 +1,10 @@
+openapi: 3.2.0
+info:
+  title: API
+  version: 1.0.0
+components:
+  requestBodies:
+    content:
+      multipart/mixed:
+        encoding: {}
+        prefixEncoding: {}