Fix OpenAPI schema ID collisions for duplicate type names

[Bellambharath]

Summary

1. Deduplicate schema reference IDs when different types resolve to the same schema ID.
2. Update CreateSchemaReferenceId tests to reflect non-nullable DateTime properties.
3. Add test case for types with the same name in different namespaces.

Testing

• dotnet test src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Microsoft.AspNetCore.OpenApi.Tests.csproj --filter "FullyQualifiedName~CreateSchemaReferenceIdTests" -p:UseIisNativeAssets=false}

Fixes #64325

[dotnet-policy-service]

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