Sync to EF 11.0.0-preview.5.26275.101

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Summary

• Sync EF Core and Microsoft.Extensions dependencies to 11.0.0-preview.5.26275.101.
• Update test infrastructure for EF's xUnit v3 migration, including Microsoft.Testing.Platform opt-in and xUnit v3 conditional trait handling.
• Add package source mapping for the new Microsoft.DotNet.XUnitV3Extensions dependency from dotnet-eng.
• Adjust provider/test fallout from the sync: preserve test culture across xUnit v3 worker threads, avoid duplicate inherited fact/theory attributes on overrides, and cast translated Math.Sign results correctly.

EF-side PRs requiring EFCore.PG updates

• dotnet/efcore@ec063d1 - Upgrade to XUnit v3
• [main] Source code updates from dotnet/dotnet dotnet/efcore#38314 - Update dependencies from build 315684

Validation

• dotnet build --no-restore passed.
• dotnet test --no-build -- --no-progress passed against local PostgreSQL (Server=localhost;Username=npgsql_tests;Password=npgsql_tests): 28,654 passed, 222 skipped.

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