chore(sonar): use concrete return types where binary-safe (CA1859)

CA1859 has no `dotnet format` batch fixer, so applied manually after
verifying each site is private/internal/test (no public-surface impact):

- Markdown.TryParseLogicalOperation: Expression? -> BoolExpr?  (private)
- Markdown.TryParseComparison:       Expression? -> Comparison? (private)
- RuleSet.GetAllSingleRules(IGroup): IEnumerable<SingleRule> -> List<SingleRule> (private overload)
- SqlExpressionClause.SplitOnComma:  IEnumerable<string> -> List<string> (private)
- QueryBreakdownRepositoryTests._repository: IQueryBreakdownRepository -> QueryBreakdownRepository (test private field)
- UnitTest1.StartsWith(...,string):  Expression -> MethodCallExpression (test private helper)

The public `RuleSet.GetAllSingleRules()` overload still returns
IEnumerable<SingleRule> — only the private recursive helper was tightened.

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

src/Strata.SqlTools.SqlBreakdown/Classes/SqlExpressionClause.cs

@@ -85,7 +85,7 @@ public class SqlExpressionClause : SqlClause, ISqlExpressionClause
     /// </summary>
     /// <param name="clause">The clause to split.</param>
     /// <returns>An enumerable collection of individual items.</returns>
-    private static IEnumerable<string> SplitOnComma(string clause)
+    private static List<string> SplitOnComma(string clause)
     {
         var items = new List<string>();
         var current = new StringBuilder();