Flag a discriminator property that is defined as an extensible enum

`oav` should issue an error message for a property that is defined as the discriminator for a schema when it is an extensible enum, I.e.`x-ms-enum: { modelAsString: true }`, because changing the set of values for a discriminator is a breaking change.

No file or test is named. Start by locating oav's schema-validation path for discriminator properties and its handling of x-ms-enum with modelAsString: true. Done means this combination produces an error because changing discriminator values is breaking, while unaffected discriminator cases retain their existing behavior.

Written by the indexing model from the issue text.