[STJ] Disallow property names that conflict with metadata property names

[eiriktsarpalis]

I think the best course of action is to disallow properties that conflict with type discriminators. It produces JSON with duplicate properties that cannot be roundtripped -- the failure of the schema exporter is merely a side-effect of configuration being invalid.

Originally posted by @eiriktsarpalis in dotnet/aspnetcore#56575 (comment)

This should include conflicting type discriminator id's, as well as reference types containing $id or $ref in cases where reference preservation has been enabled.

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[gregsdennis]

Does the usage of $id or $ref have anything to do with JSON Schema, or is it independent/other functionality?

[eiriktsarpalis]

Unrelated, it's metadata being emitted when serializing with ReferenceHandler.Preserve.

[gregsdennis]

Okay. I understand it's existing behavior, but the overlap with JSON Schema is a bit concerning for me.