Cosmos: Stop generating discriminator clause when not needed

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Note: this PR is based on top of #34143, review last commit only

@AndriySvyryd this doesn't do validation (which is also part of #20268), we can split that out or if if you feel like it, just push it into this PR?

Closes #20268

[AndriySvyryd]

@AndriySvyryd this doesn't do validation (which is also part of #20268), we can split that out or if you feel like it, just push it into this PR?

The validation should be pretty simple, just add it

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Validate all types mapped to a collection have the same configuration

Which configuration exactly?

[AndriySvyryd]

Which configuration exactly?

IsDiscriminatorMappingComplete. It doesn't make sense if it's not set to the same value on all entity types sharing the container.

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@AndriySvyryd done

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Thanks for the quick review @AndriySvyryd, will need to merge the base PRs first before I can do this one.