feat: add DEX job common infrastructure

Add foundational types for DEX job execution:

Constants:
- ErrorAction enum (Ignore, Stop) for error handling strategy

DTOs:
- CheckQueryResult: check query execution result with ReturnValue
- MainQueryResult: main query execution result with ReturnValue

Interfaces:
- ISQLExecutor: abstraction for SQL query execution and DTO mapping

All types include comprehensive XML documentation

ECMJobRunner.Application/Common/Dtos/MainQueryResult.cs

@@ -0,0 +1,16 @@
+using System.ComponentModel.DataAnnotations.Schema;
+
+namespace ECMJobRunner.Application.Common.Dtos
+{
+    /// <summary>
+    /// Result of a main query execution
+    /// </summary>
+    public class MainQueryResult
+    {
+        /// <summary>
+        /// Return value from the main query (expected: null for success)
+        /// </summary>
+        [Column("Return Value")]
+        public int? ReturnValue { get; set; }
+    }
+}