Bump Testcontainers to 4.15.0 to fix NU1903 restore failure (#315)

Testcontainers 4.12.0 depends on SSH.NET 2025.1.0, which now has two
high severity advisories (GHSA-q939-rpr3-3284, GHSA-mggc-4xg6-vcxf).
With TreatWarningsAsErrors enabled, NuGet's audit fails every
solution-wide dotnet restore, so all CI jobs fail. Testcontainers
4.14.0+ depends on the patched SSH.NET 2026.0.0.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Tests.E2e/Tests.E2e.csproj

@@ -16,7 +16,7 @@
         <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.6.0" />
         <PackageReference Include="Microsoft.Playwright" Version="1.60.0" />
         <PackageReference Include="Microsoft.Playwright.Xunit" Version="1.60.0" />
-        <PackageReference Include="Testcontainers" Version="4.12.0" />
+        <PackageReference Include="Testcontainers" Version="4.15.0" />
         <PackageReference Include="xunit" Version="2.9.3"/>
         <PackageReference Include="xunit.runner.visualstudio" Version="3.1.5">
             <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>