chore: enable TreatWarningsAsErrors and fix existing warnings

[andregoepel]

Summary

• TreatWarningsAsErrors=true plus a WarningsNotAsErrors exception for low/moderate NuGet audit findings (NU1901/NU1902) in both the root and samples/Directory.Build.props (the samples props doesn't import the root — it's applied independently). High/critical findings (NU1903/NU1904) stay errors.
• Fixed the 91 pre-existing warnings this surfaced: 88x CS8625 — test helpers building an NSubstitute proxy of UserManager<User>/SignInManager<User> passed bare null for unused non-nullable constructor parameters; followed the existing null! convention already used in AuthenticatedUserContext.cs rather than inventing a new pattern. 2x xUnit1051 and 1x CS8602 fixed the same way as the other repos in this rollout.

Test plan

• dotnet build --no-incremental (full solution incl. samples) — 0 Warning(s), 0 Error(s)
• dotnet test — AndreGoepel.Marten.Identity.Tests 191/191, AndreGoepel.Marten.Identity.Blazor.Tests 130/130 passed
• AndreGoepel.Marten.Identity.IntegrationTests — not run, Docker daemon wasn't available in the dev environment this was built in. Needs a local run with Docker up before merging.
• AndreGoepel.Marten.Identity.E2ETests — not run (own CI workflow), but compiles cleanly with 0 warnings as part of the full build
• dotnet list package --vulnerable --include-transitive — no findings

Closes #150