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>

tests/Strata.SqlTools.EFCore.Tests/QueryBreakdownRepositoryTests.cs

@@ -11,7 +11,7 @@ public class QueryBreakdownRepositoryTests
 {
     private DbContext _dbContext = null!;
     private IQueryBreakdownMapper _mapper = null!;
-    private IQueryBreakdownRepository _repository = null!;
+    private QueryBreakdownRepository _repository = null!;
 
     [SetUp]
     public void Setup()

tests/Strata.SqlTools.Rules.Tests/UnitTest1.cs

@@ -148,7 +148,7 @@ public class Tests
         return result;
     }
 
-    static System.Linq.Expressions.Expression StartsWith(System.Linq.Expressions.Expression expression, string value)
+    static MethodCallExpression StartsWith(System.Linq.Expressions.Expression expression, string value)
     {
         return StartsWith(expression, LinqExpression.Constant(value));
     }