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.PostgreSql.Tests/PostgreSql/QueryBreakdownCollectionTests.cs

@@ -22,7 +22,7 @@ public class QueryBreakdownCollectionTests
         var collection = new QueryBreakdownCollection();
 
         // Assert
-        Assert.That(collection.Count, Is.EqualTo(0));
+        Assert.That(collection, Is.Empty);
     }
 
     #endregion
@@ -56,7 +56,7 @@ public class QueryBreakdownCollectionTests
         _collection.Add(query3);
 
         // Assert
-        Assert.That(_collection.Count, Is.EqualTo(3));
+        Assert.That(_collection, Has.Count.EqualTo(3));
     }
 
     #endregion
@@ -285,7 +285,7 @@ public class QueryBreakdownCollectionTests
         // Assert
         Assert.That(sql, Does.Contain("SELECT"));
         Assert.That(sql, Does.Contain(";"));
-        Assert.That(_collection.Count, Is.EqualTo(3));
+        Assert.That(_collection, Has.Count.EqualTo(3));
     }
 
     #endregion