fix: switches to a getter for API consistency

Author: baywet

src/Microsoft.OpenApi/Expressions/CompositeExpression.cs

@@ -13,12 +13,12 @@ namespace Microsoft.OpenApi
     public class CompositeExpression : RuntimeExpression
     {
         private readonly string template;
-        private Regex expressionPattern = new(@"{(?<exp>\$[^}]*)");
+        private readonly Regex expressionPattern = new(@"{(?<exp>\$[^}]*)");
 
         /// <summary>
         /// Expressions embedded into string literal
         /// </summary>
-        public List<RuntimeExpression> ContainedExpressions = new();
+        public List<RuntimeExpression> ContainedExpressions { get; } = new();
 
         /// <summary>
         /// Create a composite expression from a string literal with an embedded expression

test/Microsoft.OpenApi.Tests/PublicApi/PublicApi.approved.txt

@@ -53,8 +53,8 @@ namespace Microsoft.OpenApi
     }
     public class CompositeExpression : Microsoft.OpenApi.RuntimeExpression
     {
-        public System.Collections.Generic.List<Microsoft.OpenApi.RuntimeExpression> ContainedExpressions;
         public CompositeExpression(string expression) { }
+        public System.Collections.Generic.List<Microsoft.OpenApi.RuntimeExpression> ContainedExpressions { get; }
         public override string Expression { get; }
     }
     public class CurrentKeys