Revert "Remove Microsoft.DotNet.XUnitAssert (#16734)"

[Copilot]

Reverts #16734, which removed the Microsoft.DotNet.XUnitAssert AOT-compatible xunit assert fork in favor of the official xunit.v3.assert.aot package.

• Restores src/Microsoft.DotNet.XUnitAssert/ (source, tests, README)
• Restores Microsoft.DotNet.XUnitAssert project entry in Arcade.slnx
• Reverts XUnit.targets back to $(UseDotNetXUnitAssert) / Microsoft.DotNet.XUnitAssert package references
• Restores XUnitAssert update instructions in Documentation/update-xunit.md
• Resolves conflicts from follow-up cleanup commit Remove leftover DotNetXUnitAssert references #16754 (same files) by restoring the pre-Remove Microsoft.DotNet.XUnitAssert #16734 state

To double check:

• The right tests are in and the right validation has happened. Guidance: https://github.com/dotnet/arcade/blob/main/Documentation/Validation.md

[Youssef1313]

I think this might not be very needed after #16763 is now merged? Just that runtime will have to use UseXUnitAOTAssert where needed (in the places where it previously have used DotNetXUnitAssert)