Fixes #4806. Fix nullable warnings and suppress xunit analyzer bug

[Copilot]

Fixes

PR #4788 (xUnit v3 upgrade) introduced CS8600 nullable warnings and AD0001 Roslyn analyzer crashes.

Proposed Changes/Todos

• Fix 21 CS8600 warnings: Record.Exception() returns Exception?, but was assigned to non-nullable Exception
• Suppress AD0001 in test projects — Xunit.Analyzers.MemberDataShouldReferenceValidMember throws InvalidOperationException (known xunit bug)

CS8600 fix (10 test files):

// Before
Exception exception = Record.Exception (() => view.SetFocus ());

// After
Exception? exception = Record.Exception (() => view.SetFocus ());

AD0001 suppression (both test .csproj files):

<NoWarn>$(NoWarn);AD0001</NoWarn>

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
• My code follows the style guidelines of Terminal.Gui - if you use Visual Studio, hit CTRL-K-D to automatically reformat your files before committing.
• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit
• I have made corresponding changes to the API documentation (using /// style comments)
• My changes generate no new warnings
• I have checked my code and corrected any poor grammar or misspellings
• I conducted basic QA to assure all features are working

Original prompt

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This section details on the original issue you should resolve

<issue_title>Warnings were introduced in #4788</issue_title>
<issue_description>- #4788

These need to be fixed. The rosyln one can be supressed because it's a bug in xunit.

I don't understand why the others were not being emitted before. But they seem to be valid nullable warnings and the code should be fixed.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Warnings were introduced in #4788 #4806

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