[BenjaminMichaelis]

Summary

Removes null-forgiving operators (!) where flow analysis, a checked throw, or the TUnit IsNotNull() return value can express the invariant. No behaviour change in production paths.

Replacement patterns

Pattern	Where
Redundant ! removed (flow analysis already proves non-null)	HomeController (siteMapping), ServiceCollectionExtensions.ResolveLocalEndpoint
Pattern/guard on the field instead of a pre-computed bool	BookSearchTool (_SearchService is not null)
?? throw new InvalidOperationException("...")	BookContentTool, AISearchService (MaxBy), ServiceCollectionExtensions (connection string), RequiresLinuxContainersAttribute (Process.Start; already caught by the existing InvalidOperationException filter)
?? string.Empty	ReferralTrackingMiddleware (QueryString.Value)
Use value returned by await Assert.That(x).IsNotNull()	McpApiTokenServiceTests, LocalChatServiceTests, PwnedPasswordValidatorTests

! intentionally kept

• EF Core DbSet<T> ... = null! in EssentialCSharpWebContext (framework-assigned).
• get => _Input!; in 14 Identity Razor Pages (model-bound [BindProperty] backing field; framework-assigned).
• null! arguments in tests that intentionally pass null to exercise argument validation or satisfy mocked constructors (PwnedPasswordTests, PwnedPasswordValidatorTests, SiteMappingTests, AIChatServiceModerationTests).

Verification

• dotnet build EssentialCSharp.Web.slnx -c Release /p:AccessToNugetFeed=false: 0 warnings, 0 errors
• EssentialCSharp.Web.Tests: 202 passed
• EssentialCSharp.Chat.Tests: 41 passed