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.Snowflake.Tests/Snowflake/RecursiveCTETests.cs

@@ -175,7 +175,7 @@ public class RecursiveCTETests
 
         // Assert
         // Snowflake stores parameters with both @ and : formats
-        var statusKey = merged.Keys.FirstOrDefault(k => k.Contains("Status") && (k.StartsWith("@") || k.StartsWith(":")));
+        var statusKey = merged.Keys.FirstOrDefault(k => k.Contains("Status") && (k.StartsWith('@') || k.StartsWith(':')));
         Assert.That(statusKey, Is.Not.Null);
         Assert.That(merged[statusKey], Is.EqualTo("active"), "Main query parameter should take precedence");
     }
@@ -247,8 +247,8 @@ public class RecursiveCTETests
 
         // Assert
         Assert.That(sql, Contains.Substring("WITH RECURSIVE"));
-        Assert.That(sql.Contains("base AS"), "Non-recursive CTE should be included");
-        Assert.That(sql.Contains("tree_hierarchy AS"), "Recursive CTE should be included");
+        Assert.That(sql, Does.Contain("base AS"), "Non-recursive CTE should be included");
+        Assert.That(sql, Does.Contain("tree_hierarchy AS"), "Recursive CTE should be included");
     }
 
     [Test]