Do not return UnparsedObject when matching nested oneOf schema

[ddadamhooper]

What does this PR do?

Removes some UnparsedObjects that come from oneOf.

I found that #4062 was failing CI because of an underlying reason: if a oneOf contains a "nested" oneOf, the nested one is always marked as "parsed" even if it isn't. The outcome: any oneOf that itself contained a (direct) unparsed oneOf branch was being parsed as UnparsedObject.

(This seems backwards: why does an inner "faux-parsed" oneOf branch mean the outer oneOf fails? Isn't "parsed" going to give false positives, not false negatives? No: a oneOf must match exactly one branch; if we accidentally mark the faux-parsed oneOf branch as parsed, then two branches are marked as parsed and the outer oneOf is therefore unmatched.)

Why isn't a oneOf marked as unparsed when no branch matches? Because we try to be forward-compatible with future objects (see #944). But it turns out we've been shipping UnparsedObject for years. I've unit-tested a 2023 class (introduced in #1843) as part of the fix.

Additional Notes

It's hard to choose where to draw the line here -- which part of a request should be marked UnparsedObject and which part should succeed? I didn't try to formalize it; I just focused on ensuring that some valid objects get marked as parsed where they weren't before.

Review checklist

Please check relevant items below:

• This PR includes all newly recorded cassettes for any modified tests.
• This PR does not rely on API client schema changes.
  • The CI should be fully passing.
• Or, this PR relies on API schema changes and this is a Draft PR that includes tests.
  • Note: CI shouldn't be run on this Draft PR, as its expected to fail without the corresponding schema changes.

[ddadamhooper]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Bravo.

Reviewed commit: f03b2c0fc7

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