Add PropertyNames property to EntityBaseOptions

Introduce PropertyNames as a read-only property exposing the keys of ColumnMappings. Refactor EnsureProperties to use PropertyNames for improved clarity and maintainability.

src/ReC.Infrastructure/Options/Shared/EntityBaseOptions.cs

@@ -6,9 +6,11 @@ public record EntityBaseOptions()
 {
     public Dictionary<string, string> ColumnMappings { get; init; } = [];
 
+    public IEnumerable<string> PropertyNames => ColumnMappings.Select(col => col.Key);
+
     public void EnsureProperties(IEnumerable<string> propertyNames)
     {
-        var missingProperties = propertyNames.Except(ColumnMappings.Select(col => col.Key)).ToList();
+        var missingProperties = propertyNames.Except(PropertyNames).ToList();
 
         if (missingProperties.Count != 0)
             throw new InvalidOperationException($"The following properties are not configured: {string.Join(", ", missingProperties)}");