Description

This PR backports #57972 to resolve issues with concurrent requests to OpenAPI documents (fixes #57876).

This PR also addresses a set of issues with generated schemas:

• Resolves an issue where self-referential schemas were not generated correctly for IEnumerable-types (fixes OpenAPI document generation does not generate correct refs for self-referencing collection properties #58006)
• Resolves an issue where enum types in query parameters were not resolved to the correct model metadata type for MVC action APIs (fixes OpenApi inline enum for query parameters #57979)
• Resolves an issue where JsonUnmappedMemberHandling.Disallow did not work as expected because the underlying JsonNode was not mapped correctly (fixes UnmappedMemberHandling not working with Microsoft.AspNetCore.OpenApi #57981)

Customer Impact

This PR resolves a set of bugs reported by users of .NET 9 RC 1 for which no viable workarounds exist.

Regression?

• Yes
• No

Risk

• High
• Medium
• Low

Changes localized to OpenAPI package.

Verification

• Manual (required)
• Automated

Packaging changes reviewed?

• Yes
• No
• N/A