[Inline model resolver] various improvements

[wing328]

• better handling of request bodies
• better inline model naming (avoid using inline_object if possible)
• fix unique naming

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[wing328]

@impl I wonder if you can review this change when you've time as you've filed a similar change. Thank you.

[impl]

Hi @wing328,

This does appear to solve the same problem I was looking to address with #12129, with the additional benefit of better naming for inline models as well. Thank you! I'll close that one in favor of this PR. LGTM!

[wing328]

@TiFu FYI. We've updated the inlined resolver to create a separate model/schema for all inline schemas(including oneOf/allOf) in the request bodies.