fix: Pydantic 2 being more strict about type validation

With Pydantic v2, strict type validation is enforced. The type field is expected
to have the exact value "POST" as a literal.

tests/test_post.json

@@ -1 +1,4 @@
-{"hello": "world"}
+{
+    "type": "POST",
+    "hello": "world"
+}