feat: adds support for populating OpenAPI discriminator defaultMapping

[baywet]

fixes #65724

CC @martincostello

[dotnet-policy-service]

Thanks for your PR, @baywet. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[Copilot]

Pull request overview

Adds OpenAPI 3.2 discriminator defaultMapping generation for polymorphic schemas when the non-abstract base type explicitly includes itself as a derived type (via JsonDerivedType), aligning the emitted schema with OpenAPI 3.2 discriminator semantics.

Changes:

• Emit discriminatorDefaultMapping during JSON schema export when the base type is non-abstract and includes a self JsonDerivedType with a discriminator.
• Parse discriminatorDefaultMapping from the intermediate JSON schema representation and resolve discriminator references into document components.
• Extend the polymorphism OpenAPI schema service test to validate Discriminator.DefaultMapping is present and resolves to the base schema.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Services/OpenApiSchemaService/OpenApiSchemaService.PolymorphicSchemas.cs	Adds assertions validating Discriminator.DefaultMapping resolves to the base Pet schema.
src/OpenApi/src/Services/Schemas/OpenApiSchemaService.cs	Resolves discriminator mapping/defaultMapping schema references against the OpenAPI document components.
src/OpenApi/src/Schemas/OpenApiSchemaKeywords.cs	Introduces a keyword constant for discriminator default mapping metadata.
src/OpenApi/src/Schemas/OpenApiJsonSchema.Helpers.cs	Adds deserialization support for discriminatorDefaultMapping into OpenApiDiscriminator.DefaultMapping.
src/OpenApi/src/Extensions/JsonNodeSchemaExtensions.cs	Emits discriminatorDefaultMapping when appropriate for non-abstract base types that self-reference as a derived type with a discriminator.

[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated no new comments.

[Youssef1313]

@baywet There are test failures here:

• OpenApiDocumentLocalizationTests.VerifyOpenApiDocumentIsInvariant [FAIL]
• OpenApiDocumentIntegrationTests.OpenApiDocumentIsValid(documentName: "schemas-by-ref", version: OpenApi3_1) [FAIL]
• OpenApiDocumentIntegrationTests.OpenApiDocumentIsValid(documentName: "schemas-by-ref", version: OpenApi3_2) [FAIL]