[Rule] Allowed values for an enum have changed

Enum “foo” had allowed values as “val1” and “val2” in v1 of API. If now, the values allowed/accepted by the service are “val1”, “val2” and “val3”, client will fail to de-serialize if “val3” comes back in the response.

Start by reproducing the issue with two OpenAPI specifications where enum “foo” gains “val3”, then trace the command-line tool’s rule comparison for allowed enum values. Done means the change is reported when a newer specification accepts a value that the earlier one did not, covering the described deserialization risk.

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