Support OpenAPI 3.2

[alchemax-housekeeper]

OpenAPI 3.2 has been released upstream (see the OpenAPI Specification releases), but is not yet supported by oapi-codegen-rust.

Note

This issue is not auto-assigned (GitHub App tokens can't assign Copilot as an agent). A maintainer should assign it to @copilot manually once it's ready to be picked up.

Please do a thorough deep dive into what it would take to support OpenAPI 3.2 here:

• Summarise the notable specification changes in OpenAPI 3.2 relative to the versions we already support.
• Identify the concrete parts of the implementation (loader, lowering, IR, emitters, config, tests) that would need to change.
• Call out the biggest challenges, ambiguities, and design decisions to be made.
• Propose a rough, staged plan of action to reach parity, in line with how previous versions were added.

Filed automatically by the OpenAPI version check workflow.