Enable TreatWarningsAsErrors repo-wide and fix 191 build warnings

[mattleibow]

Summary

The solution currently produces 191 build warnings. We should enable TreatWarningsAsErrors in Directory.Build.props and fix or suppress these warnings to prevent regressions.

DeviceRunners.VisualRunners.Core already has TreatWarningsAsErrors=true (added in PR #111).

Warning Breakdown

Project	Count	Warnings
DeviceRunners.VisualRunners.Maui	~143	XamlC XC0022 — uncompiled bindings (add x:DataType to XAML)
DeviceRunners.VisualRunners	34	CS8602 (null deref ×14), CS8769 (nullability mismatch ×12), CS8622 (nullability delegate ×6), CS8603 (possible null return ×2)
DeviceTestingKitApp.DeviceTests	8	Xamarin.Shared.Sdk PublishFolderType metadata warnings, CS8892 (dual entry point)
DeviceRunners.Cli	4	NU1510 — unnecessary System.Security.Cryptography.X509Certificates PackageReference
DeviceRunners.slnx	2	NU1510

Suggested Approach

1. Fix the 34 nullability warnings in DeviceRunners.VisualRunners
2. Add x:DataType to XAML bindings in DeviceRunners.VisualRunners.Maui (or suppress XC0022)
3. Remove unnecessary System.Security.Cryptography.X509Certificates package from CLI
4. Address sample project warnings
5. Enable <TreatWarningsAsErrors>true</TreatWarningsAsErrors> in Directory.Build.props