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/MainQueryExecutionBehavior.cs

@@ -0,0 +1,113 @@
+using ECMJobRunner.Application.Common.Constants;
+using ECMJobRunner.Application.Common.Dtos;
+using ECMJobRunner.Application.Common.Exceptions;
+using ECMJobRunner.Domain.Interfaces;
+using ECMJobRunner.Application.Common.Options;
+using ECMJobRunner.Application.DEXJob;
+using MediatR;
+using Microsoft.Extensions.Options;
+using System;
+using System.Text.RegularExpressions;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace ECMJobRunner.Application.Behaviors
+{
+    /// <summary>
+    /// Pipeline behavior that executes the main SQL query for DEX jobs
+    /// Runs before the command handler and validates query results
+    /// </summary>
+    /// <typeparam name="TRequest">The request type</typeparam>
+    /// <typeparam name="TResponse">The response type</typeparam>
+    public class MainQueryExecutionBehavior<TRequest, TResponse> : IPipelineBehavior<TRequest, TResponse>
+        where TRequest : notnull
+    {
+        private readonly ISQLExecutor _executor;
+        private readonly DexJobOptions _options;
+
+        /// <summary>
+        /// Initializes a new instance of MainQueryExecutionBehavior
+        /// </summary>
+        /// <param name="executor">SQL executor for query execution</param>
+        /// <param name="options">DEX job configuration options</param>
+        public MainQueryExecutionBehavior(ISQLExecutor executor, IOptions<DexJobOptions> options)
+        {
+            _executor = executor;
+            _options = options.Value;
+        }
+
+        /// <summary>
+        /// Handles the pipeline behavior
+        /// Executes main query 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 ExecuteMainQueryAsync(command, cancellationToken);
+            }
+
+            return await next();
+        }
+
+        private async Task ExecuteMainQueryAsync(TriggeringDEXJobCommand command, CancellationToken cancel)
+        {
+            if (!string.IsNullOrWhiteSpace(command.Job.SqlMainQuery))
+            {
+                var sqlMainQuery = Regex.Replace(
+                    command.Job.SqlMainQuery!,
+                    _options.Placeholders.BatchId.Pattern,
+                    command.BatchId,
+                    _options.Placeholders.BatchId.RegexOptions);
+
+                try
+                {
+                    var result = await _executor.ExecuteQueryAsync<MainQueryResult>(sqlMainQuery, cancel);
+                    if (_options.Error.MainQuery.OnUnexpectedResult == ErrorAction.Stop)
+                    {
+                        if (result is null)
+                            throw new JobSqlException(
+                                jobName: "Triggering DEX",
+                                processName: "Main Query",
+                                batchId: command.BatchId,
+                                reason: "Main Query returned nothing.",
+                                query: sqlMainQuery,
+                                innerException: null);
+                        else if (result.ReturnValue is not null)
+                            throw new JobSqlException(
+                                jobName: "Triggering DEX",
+                                processName: "Main Query",
+                                batchId: command.BatchId,
+                                reason: $"The query unexpectedly returned the value {result.ReturnValue}. The expected value was null.",
+                                query: sqlMainQuery, innerException: null);
+                    }
+                }
+                catch (Exception ex)
+                {
+                    if (_options.Error.MainQuery.OnExecution == ErrorAction.Stop)
+                        throw new JobSqlException(
+                            jobName: "Triggering DEX",
+                            processName: "Main Query",
+                            batchId: command.BatchId,
+                            reason: null,
+                            query: sqlMainQuery,
+                            innerException: ex);
+                }
+            }
+            else if (_options.Error.MainQuery.IfNullOrWhiteSpace == ErrorAction.Stop)
+            {
+                throw new JobSqlException(
+                    jobName: "Triggering DEX",
+                    processName: "Main Query",
+                    batchId: command.BatchId,
+                    reason: "SQL Check Query is null or empty",
+                    query: null,
+                    innerException: null);
+            }
+        }
+    }
+}