Flatten inline schemas in requestBody components

[impl]

For request bodies that are completely inlined into a path's operations, the inline model resolver correctly generates both a schema component and a requestBody component and adds $refs.

However, when an operation already contains an $ref to a requestBody that itself contains an inline schema, that schema was not being flattened to a separate component. This change adds a separate method to do such flattening and add an $ref if needed.

This fixes the dreaded error about a model having an unknown base type (with typically invalid code generated) in this situation.

I have included several tests to verify the corrected behavior, including one test each for the issues this fixes (I suspect there are more duplicates, but this is what I could find from an initial search). I know the issue-specific tests are a little redundant, but I wanted to make sure each was actually solved, as everyone's specs varied slightly.

Fixes #8605
Fixes #9123
Fixes #10435
Fixes #11811

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[wing328]

@impl thanks for the PR. I'm also refactoring the inline model resolver and will review this PR shortly (or include this change in my PR for refactoring)

[impl]

Closing in favor of #12293.