Discriminator Mapping Support for oneOf about openapi-typescript HOT 3 OPEN

I had a good look at the code now and read the discriminator unit tests. This case of oneOf seems to be not supported. There is a oneOf case that is tested, but it doesn't seem to even use a discriminator, the OpenAPI input is invalid and the output is simply a oneOf union:

The default usecase for oneOf discriminators seems to be not handled right now, so I changed the title of this feature request accordingly.

I also have a proposal now and might be able to implement this. The hard part is searching for all discriminator properties referenced by a discriminator. This should be done via the mapping itself, where we already get the path of the schema object. If the object consists of an allOf (for example a base object for the discriminator, where the discriminator property really is), the allOf needs to be resolved. Finally we need to overwrite the discriminator property with the single mapped value to mimick the Typescript union discimination.

We probably need to enhance the discriminator scan and add the discriminator property paths to the ctx information. That way we should be able to quickly find the discriminator properties inside transformSchemaObjectCore and transform { type: 'string', enum: [value] } instead of the full enum.

That doesn't take the resolved allOfs into account. Maybe we can append the discriminator property to the mapped schema objects to overwrite any inherited discriminator properties?

@drwpow Shall I try to implement the easy case from the OpenAPI above? It is the default case for oneOf from the OpenAPI documentation for discriminators.

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I PR'd a proposal so we can discuss the new support: #1574

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#1574 was merged into main now. Working on backporting to 6.x now.

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