Merge PR #3477: align C# rule with AwesomeAssertions guidance

rules/csharp/testing.md

@@ -11,7 +11,7 @@ paths:
 ## Test Framework
 
 - Prefer **xUnit** for unit and integration tests
-- Use **FluentAssertions** for readable assertions
+- Use **AwesomeAssertions** for readable assertions
 - Use **Moq** or **NSubstitute** for mocking dependencies
 - Use **Testcontainers** when integration tests need real infrastructure