[UIKit] Enable nullability and clean up UIAppearance.

[rolfbjarne]

This is file 4 of 30 files with nullability disabled in UIKit.

• Enable nullability (#nullable enable).
• Remove unused System.Reflection using directive.
• Use ArgumentNullException.ThrowIfNull() instead of manual null checks.
• Add nullable annotations (UIAppearance?, object?).
• Improve XML documentation comments: remove 'To be added.' placeholders, fix whitespace, add missing docs for operators and GetAppearance methods, add 'see cref' references.

Contributes towards #17285.

[Copilot]

Pull request overview

Enables nullable reference types for UIKit.UIAppearance and updates related documentation/test baselines to reflect the resulting API/docs output changes.

Changes:

• Enabled #nullable in src/UIKit/UIAppearance.cs and updated signatures/null-checks accordingly.
• Improved XML documentation for UIAppearance members (operators and GetAppearance overloads).
• Updated documentation known-failures and iOS size-test expected outputs to match the new build artifacts.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tests/dotnet/UnitTests/expected/iOS-NativeAOT-size.txt	Updates expected NativeAOT size baselines to match new output.
tests/dotnet/UnitTests/expected/iOS-MonoVM-size.txt	Updates expected MonoVM size baselines to match new output.
tests/dotnet/UnitTests/expected/iOS-MonoVM-interpreter-size.txt	Updates expected MonoVM interpreter size baselines to match new output.
tests/cecil-tests/Documentation.KnownFailures.txt	Removes UIAppearance operator doc entries from the known-failures list (now documented).
src/UIKit/UIAppearance.cs	Enables nullability, refines null checks, and improves XML docs for equality operators and GetAppearance overloads.