fix(library): handle circular schema references

[baywet]

Summary

• Handle circular schema references without failing document loading.
• Add regression coverage for circular schema references.

Testing

• dotnet test .\test\Microsoft.OpenApi.Readers.Tests\Microsoft.OpenApi.Readers.Tests.csproj --no-restore --no-logo
• dotnet test .\test\Microsoft.OpenApi.Tests\Microsoft.OpenApi.Tests.csproj --no-restore --no-logo

[baywet]

@copilot refresh the benchmark reports, change nothing else

[Copilot]

@copilot refresh the benchmark reports, change nothing else

Done in 18637f9 — benchmark reports have been refreshed, and no other files were changed.

[sonarqubecloud]

Quality Gate failed

Failed conditions
6.2% Duplication on New Code (required ≤ 3%)

See analysis details on SonarQube Cloud