feat(library): add missing json schema properties

[Vincent Biret (baywet)]

Summary

• add IOpenApiSchemaMissingProperties as a compatibility surface for the remaining JSON Schema 2020-12 schema keywords without breaking IOpenApiSchema
• wire the new schema properties through OpenApiSchema, OpenApiSchemaReference, versioned readers, and 3.0 x-oai-* compatibility serialization
• fix WriteJsonSchemaKeywords so nested schema-valued keywords serialize with the target version callback

Testing

• dotnet build src\Microsoft.OpenApi\Microsoft.OpenApi.csproj --nologo
• dotnet test test\Microsoft.OpenApi.Tests\Microsoft.OpenApi.Tests.csproj --nologo --no-build --filter "OpenApiSchemaTests|OpenApiSchemaReferenceTests|OpenApiSchemaSerializationTests"
• dotnet test test\Microsoft.OpenApi.Readers.Tests\Microsoft.OpenApi.Readers.Tests.csproj --nologo --filter "OpenApiSchemaTests"

Closes #2877
Closes #2878
Closes #2879

[github-code-quality]

Code Coverage Overview

Languages: C#

C# / code-coverage/dotnet

The overall coverage in the feat/missing-json-sc... branch remains at 88%, unchanged from the main branch.

Show a code coverage summary of the most impacted files.

File	main 21488c6	feat/missing-json-sc... 9cf0916	+/-
/home/runner/wo...emaReference.cs	93%	93%	0%
/home/runner/wo...penApiSchema.cs	99%	99%	0%
/home/runner/wo...Deserializer.cs	87%	90%	+3%
/home/runner/wo...Deserializer.cs	87%	90%	+3%
/home/runner/wo...Deserializer.cs	81%	86%	+5%

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_{Updated June 09, 2026 17:57 UTC
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[sonarqubecloud]

Quality Gate failed

Failed conditions
8.8% Duplication on New Code (required ≤ 3%)

See analysis details on SonarQube Cloud