Correctly match oneOf mappings when generating example values

[Brads3290]

Correctly match oneOf mappings when generating example values for oneOf's with discriminators.
Previously it was trying to match the mapping against the parent entity (the one with the oneOf set), which would fail to match, therefore excluding the discriminator property (#8336)

Motivation and Context

Fixes #8336

How Has This Been Tested?

Manually tested to see that the discriminator property now shows up

Checklist

My PR contains...

• No code changes (src/ is unmodified: changes to documentation, CI, metadata, etc.)
• Dependency changes (any modification to dependencies in package.json)
• Bug fixes (non-breaking change which fixes an issue)
• Improvements (misc. changes to existing features)
• Features (non-breaking change which adds functionality)

My changes...

• are breaking changes to a public API (config options, System API, major UI change, etc).
• are breaking changes to a private API (Redux, component props, utility functions, etc.).
• are breaking changes to a developer API (npm script behavior changes, new dev system dependencies, etc).
• are not breaking changes.

Documentation

• My changes do not require a change to the project documentation.
• My changes require a change to the project documentation.
• If yes to above: I have updated the documentation accordingly.

Automated tests

• My changes can not or do not need to be tested.
• My changes can and should be tested by unit and/or integration tests.
• If yes to above: I have added tests to cover my changes.
• If yes to above: I have taken care to cover edge cases in my tests.
• All new and existing tests passed.

[Brads3290]

Side note: I ticked "My changes can and should be tested by unit and/or integration tests." because there probably should be a test for this, but I'm not very well versed with React or the JS ecosystem in general so I couldn't even find the tests that were relevant to the area of code that I changed.

If someone wants to add the test or point me in the right direction, I would appreciate it.