chore(sonar): adopt Has.Count, Is.Empty, Does.Contain, char overloads in tests (NUnit2046, NUnit2011, CA1866)

- NUnit2046: `Assert.That(x.Count, Is.EqualTo(n))` → `Assert.That(x, Has.Count.EqualTo(n))` (or `Is.Empty` when n==0)
- NUnit2011: `Assert.That(s.Contains(x))` → `Assert.That(s, Does.Contain(x))` for richer failure messages
- CA1866: `.StartsWith("$"|"@"|":")` → `.StartsWith('$'|'@'|':')` char overload

Driven by `dotnet format analyzers --diagnostics NUnit2046 NUnit2011 CA1866`
for the cases the Roslyn fixer handles, plus a regex sweep for the remaining
`Count == n` (n>0) cases which the fixer doesn't address. CA1866 had no
associated code fix and was edited by hand (3 sites in 2 files). All tests
green (1180 passing).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/Strata.SqlTools.SqlBreakdown.Tests/Exceptions/ExceptionHandlingTests.cs

@@ -389,7 +389,7 @@ public class ExceptionHandlingTests
         query.AddWithClause("active_users", cteQuery);
 
         // Assert
-        Assert.That(query.WithClauses.Count, Is.EqualTo(1));
+        Assert.That(query.WithClauses, Has.Count.EqualTo(1));
         Assert.That(query.WithClauses[0].TableName, Is.EqualTo("active_users"));
     }