dotnet: replace FluentAssertions with AwesomeAssertions in test suite

[Copilot]

This PR updates the .NET test project to remove FluentAssertions due to its licensing change and switches to AwesomeAssertions as a compatible alternative without the same license constraints.

• Dependency migration

  • Replaced the test-project package reference from FluentAssertions to AwesomeAssertions in dotnet/Vaas/test/Vaas.Test/Vaas.Test.csproj.
  • Pinned to AwesomeAssertions 9.0.0 to align with available NuGet versions.

• Namespace updates in tests

  • Updated assertion imports across affected test files from using FluentAssertions; to using AwesomeAssertions;.
  • No production SDK code paths were modified; scope is limited to test code and test dependencies.

• Representative change

  <!-- Vaas.Test.csproj -->
  <PackageReference Include="AwesomeAssertions" Version="9.0.0" />

  // test files
  using AwesomeAssertions;