Address timestamp filter review comments

src/modules/Elsa.Alterations/Endpoints/Alterations/DryRun/Endpoint.cs

@@ -1,7 +1,6 @@
 using Elsa.Abstractions;
 using Elsa.Alterations.Core.Contracts;
 using Elsa.Alterations.Core.Models;
-using Elsa.Workflows.Management.Filters;
 using JetBrains.Annotations;
 using Microsoft.AspNetCore.Http;
 
@@ -36,11 +35,6 @@ public class DryRun(IWorkflowInstanceFinder workflowInstanceFinder) : ElsaEndpoi
 
     private bool ValidateInput(AlterationWorkflowInstanceFilter filter)
     {
-        var errors = WorkflowInstanceFilter.ValidateTimestampFilters(filter.TimestampFilters).ToList();
-
-        foreach (var error in errors)
-            AddError(error);
-
-        return errors.Count == 0;
+        return TimestampFilterValidation.Validate(filter.TimestampFilters, error => AddError(error));
     }
 }

src/modules/Elsa.Alterations/Endpoints/Alterations/Submit/Endpoint.cs

@@ -3,7 +3,6 @@ using Elsa.Alterations.Core.Contracts;
 using Elsa.Alterations.Core.Models;
 using Elsa.Common;
 using Elsa.Workflows;
-using Elsa.Workflows.Management.Filters;
 using JetBrains.Annotations;
 using Microsoft.AspNetCore.Http;
 
@@ -53,11 +52,6 @@ public class Submit : ElsaEndpoint<AlterationPlanParams, Response>
 
     private bool ValidateInput(AlterationPlanParams planParams)
     {
-        var errors = WorkflowInstanceFilter.ValidateTimestampFilters(planParams.Filter?.TimestampFilters).ToList();
-
-        foreach (var error in errors)
-            AddError(error);
-
-        return errors.Count == 0;
+        return TimestampFilterValidation.Validate(planParams.Filter?.TimestampFilters, error => AddError(error));
     }
 }

src/modules/Elsa.Alterations/Endpoints/Alterations/TimestampFilterValidation.cs

@@ -0,0 +1,20 @@
+using Elsa.Workflows.Management.Filters;
+using Elsa.Workflows.Management.Models;
+
+namespace Elsa.Alterations.Endpoints.Alterations;
+
+internal static class TimestampFilterValidation
+{
+    public static bool Validate(IEnumerable<TimestampFilter>? timestampFilters, Action<string> addError)
+    {
+        var isValid = true;
+
+        foreach (var error in WorkflowInstanceFilter.ValidateTimestampFilters(timestampFilters))
+        {
+            addError(error);
+            isValid = false;
+        }
+
+        return isValid;
+    }
+}

src/modules/Elsa.Workflows.Api/Endpoints/WorkflowInstances/List/Endpoint.cs

@@ -84,12 +84,15 @@ internal class List(IWorkflowInstanceStore store) : ElsaEndpoint<Request, Respon
             return false;
         }
 
-        var timestampFilterErrors = WorkflowInstanceFilter.ValidateTimestampFilters(request.TimestampFilters).ToList();
+        var hasTimestampFilterErrors = false;
 
-        foreach (var error in timestampFilterErrors)
+        foreach (var error in WorkflowInstanceFilter.ValidateTimestampFilters(request.TimestampFilters))
+        {
             AddError(error);
+            hasTimestampFilterErrors = true;
+        }
 
-        return timestampFilterErrors.Count == 0;
+        return !hasTimestampFilterErrors;
     }
 
     private async Task<Page<WorkflowInstanceSummary>> FindAsync(Request request, WorkflowInstanceFilter filter, PageArgs pageArgs, CancellationToken cancellationToken)

src/modules/Elsa.Workflows.Management/Filters/WorkflowInstanceFilter.cs

@@ -166,6 +166,9 @@ public class WorkflowInstanceFilter
         {
             foreach (var timestampFilter in TimestampFilters)
             {
+                if (timestampFilter == null)
+                    throw new ArgumentException("Timestamp filter must be specified.", nameof(TimestampFilters));
+
                 var column = NormalizeTimestampFilterColumn(timestampFilter.Column);
                 var timestamp = timestampFilter.Timestamp;
                 var isZeroTime = timestamp.TimeOfDay == TimeSpan.Zero;

test/integration/Elsa.Alterations.IntegrationTests/AlterationsApiTimestampFilterTests.cs

@@ -98,6 +98,19 @@ public class AlterationsApiTimestampFilterTests : IAsyncLifetime
         Assert.Contains("Timestamp filter must be specified.", body);
     }
 
+    [Theory]
+    [InlineData("/alterations/dry-run")]
+    [InlineData("/alterations/submit")]
+    public async Task Post_WithMultipleInvalidTimestampFilters_ReturnsAllValidationErrors(string path)
+    {
+        var response = await _httpClient.PostAsJsonAsync(path, CreateRequestWithMultipleInvalidTimestampFilters(path));
+        var body = await response.Content.ReadAsStringAsync();
+
+        Assert.Equal(HttpStatusCode.BadRequest, response.StatusCode);
+        Assert.Contains("Timestamp filter column must be specified.", body);
+        Assert.Contains("Timestamp filter must be specified.", body);
+    }
+
     private static object CreateRequest(string path)
     {
         var filter = new
@@ -129,4 +142,25 @@ public class AlterationsApiTimestampFilterTests : IAsyncLifetime
             ? new { filter }
             : filter;
     }
+
+    private static object CreateRequestWithMultipleInvalidTimestampFilters(string path)
+    {
+        var filter = new
+        {
+            timestampFilters = new object?[]
+            {
+                new
+                {
+                    column = " ",
+                    @operator = TimestampFilterOperator.Is,
+                    timestamp = new DateTimeOffset(2026, 5, 20, 10, 0, 0, TimeSpan.Zero)
+                },
+                null
+            }
+        };
+
+        return path == "/alterations/submit"
+            ? new { filter }
+            : filter;
+    }
 }

test/unit/Elsa.Workflows.Management.UnitTests/Filters/WorkflowInstanceFilterTimestampTests.cs

@@ -70,6 +70,20 @@ public class WorkflowInstanceFilterTimestampTests
         Assert.Contains("CreatedAt, UpdatedAt, FinishedAt", exception.Message);
     }
 
+    [Fact]
+    public void Apply_WithNullTimestampFilter_ThrowsClearArgumentException()
+    {
+        var filter = new WorkflowInstanceFilter
+        {
+            TimestampFilters = [null!]
+        };
+
+        var exception = Assert.Throws<ArgumentException>(() => filter.Apply(_workflowInstances).ToList());
+
+        Assert.Contains("Timestamp filter must be specified.", exception.Message);
+        Assert.Equal(nameof(WorkflowInstanceFilter.TimestampFilters), exception.ParamName);
+    }
+
     [Fact]
     public void ValidateTimestampFilters_WithMissingColumn_ReturnsClearValidationError()
     {