fix: restore error aggregation in ValidateRow, assert validation keys in tests

- Restore accumulation of multiple validation errors per member per row
  (regression from refactoring: was overwriting instead of appending)
- Assert ValidationProblemDetails.Errors contains Page/PageSize keys
  in invalid pagination tests

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

Author: sheepla

src/AxisEndpoints.Extensions.CsvHelper/CsvRequest.cs

@@ -168,7 +168,15 @@ Dictionary<string, string[]> errors
         {
             var memberName = result.MemberNames.FirstOrDefault() ?? "(unknown)";
             var key = $"row {rowNumber}: {memberName}";
-            errors[key] = [result.ErrorMessage ?? "Validation failed."];
+            var message = result.ErrorMessage ?? "Validation failed.";
+            if (errors.TryGetValue(key, out var existing))
+            {
+                errors[key] = [..existing, message];
+            }
+            else
+            {
+                errors[key] = [message];
+            }
         }
     }
 }

tests/AxisEndpoints.Example.Tests/ListUsersEndpointTests.cs

@@ -3,6 +3,7 @@
 using AxisEndpoints.Example.Features.Users;
 using AxisEndpoints.Example.Features.Users.List;
 using FluentAssertions;
+using Microsoft.AspNetCore.Mvc;
 
 namespace AxisEndpoints.Example.Tests;
 
@@ -76,6 +77,9 @@ public async Task ListUsers_InvalidPage_Returns400()
         var response = await _client.GetAsync("/api/users?page=0");
 
         response.StatusCode.Should().Be(HttpStatusCode.BadRequest);
+        var body = await response.Content.ReadFromJsonAsync<ValidationProblemDetails>();
+        body.Should().NotBeNull();
+        body!.Errors.Should().ContainKey("Page");
     }
 
     [Fact]
@@ -84,5 +88,8 @@ public async Task ListUsers_InvalidPageSize_Returns400()
         var response = await _client.GetAsync("/api/users?pageSize=0");
 
         response.StatusCode.Should().Be(HttpStatusCode.BadRequest);
+        var body = await response.Content.ReadFromJsonAsync<ValidationProblemDetails>();
+        body.Should().NotBeNull();
+        body!.Errors.Should().ContainKey("PageSize");
     }
 }