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.Rules/Rule/Expression/Markdown.cs

@@ -115,7 +115,7 @@ public static class Markdown
         return ParseAtomicExpression(text);
     }
 
-    private static Expression? TryParseLogicalOperation(string text)
+    private static BoolExpr? TryParseLogicalOperation(string text)
     {
         foreach (var op in LogicalOperators.Keys)
         {
@@ -130,7 +130,7 @@ public static class Markdown
         return null;
     }
 
-    private static Expression? TryParseComparison(string text)
+    private static Comparison? TryParseComparison(string text)
     {
         foreach (var op in ComparisonOperators.Keys)
         {

src/Strata.SqlTools.Rules/Rule/RuleSet.cs

@@ -50,7 +50,7 @@ public class RuleSet : IGroup
     /// </summary>
     /// <param name="group">The rule group to traverse.</param>
     /// <returns>An enumerable of all single rules found in the group.</returns>
-    private static IEnumerable<SingleRule> GetAllSingleRules(IGroup group)
+    private static List<SingleRule> GetAllSingleRules(IGroup group)
     {
         var rules = new List<SingleRule>();
         foreach (var rule in group.Rules)

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();

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));
     }