feat(application): implement MediatR pipeline behaviors for DEX job stages

- Add MainQueryExecutionBehavior
  - Execute main SQL query via ISQLExecutor
  - Populate command.MainQueryResults
  - Throw JobSqlException on failure
- Add CheckQueryExecutionBehavior
  - Execute check SQL query via ISQLExecutor
  - Validate ErrorAction.SkipInsert for zero results
  - Throw JobSqlException on failure/validation error
- Add ReCRequestExecutionBehavior
  - Execute ReC API requests for each main query result
  - Batch ID tracking, error handling
  - Throw JobHttpException on API failures
- Conditional compilation for MediatR signature differences
  - net480: Handle(request, cancellationToken, next)
  - net8.0: Handle(request, next, cancellationToken)
- Refactor TriggeringDEXJobCommand handler: delegate all logic to behaviors

ECMJobRunner.Application/Behaviors/ReCRequestExecutionBehavior.cs

@@ -0,0 +1,81 @@
+using ECMJobRunner.Application.Common.Constants;
+using ECMJobRunner.Application.Common.Exceptions;
+using ECMJobRunner.Application.Common.Options;
+using ECMJobRunner.Application.DEXJob;
+using MediatR;
+using Microsoft.Extensions.Options;
+using ReC.Client;
+using ReC.Client.Api;
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace ECMJobRunner.Application.Behaviors
+{
+    /// <summary>
+    /// Pipeline behavior that sends ReC HTTP request for DEX jobs
+    /// Runs after CheckQueryExecutionBehavior and invokes ReC API
+    /// </summary>
+    /// <typeparam name="TRequest">The request type</typeparam>
+    /// <typeparam name="TResponse">The response type</typeparam>
+    public class ReCRequestExecutionBehavior<TRequest, TResponse> : IPipelineBehavior<TRequest, TResponse>
+        where TRequest : notnull
+    {
+        private readonly ReCClient _reCClient;
+        private readonly DexJobOptions _options;
+
+        /// <summary>
+        /// Initializes a new instance of ReCRequestExecutionBehavior
+        /// </summary>
+        /// <param name="reCClient">ReC client for HTTP requests</param>
+        /// <param name="options">DEX job configuration options</param>
+        public ReCRequestExecutionBehavior(ReCClient reCClient, IOptions<DexJobOptions> options)
+        {
+            _reCClient = reCClient;
+            _options = options.Value;
+        }
+
+        /// <summary>
+        /// Handles the pipeline behavior
+        /// Sends ReC request if request is TriggeringDEXJobCommand
+        /// </summary>
+#if NET48
+        public async Task<TResponse> Handle(TRequest request, CancellationToken cancellationToken, RequestHandlerDelegate<TResponse> next)
+#else
+        public async Task<TResponse> Handle(TRequest request, RequestHandlerDelegate<TResponse> next, CancellationToken cancellationToken)
+#endif
+        {
+            if (request is TriggeringDEXJobCommand command)
+            {
+                await SendReCRequestAsync(command, cancellationToken);
+            }
+
+            return await next();
+        }
+
+        private async Task SendReCRequestAsync(TriggeringDEXJobCommand command, CancellationToken cancel)
+        {
+            try
+            {
+                await _reCClient.RecActions.InvokeAsync(command.Job.ProfileId, new InvokeReferences()
+                {
+                    BatchId = command.BatchId,
+                }, cancel);
+            }
+            catch (Exception ex)
+            {
+                if (_options.Error.ReCRequest.OnSending == ErrorAction.Stop)
+                {
+                    throw new JobHttpException(
+                        jobName: "Triggering DEX",
+                        processName: "ReC Http Request",
+                        batchId: command.BatchId,
+                        reason: null,
+                        clientLibrary: "ReC.Client",
+                        clientMethod: "RecActions.InvokeAsync",
+                        innerException: ex);
+                }
+            }
+        }
+    }
+}