VPR-59 refactor(cms): share stale-edit and permission-list helpers

Three byte-identical copies of AssertNotStale and CleanList lived in
the file, content-block, and left-nav services; extract them to
CmsServiceHelpers so the concurrency contract cannot drift.

Author: rlorenzo

web/Areas/CMS/Services/CmsFileService.cs

@@ -434,20 +434,8 @@ public async Task<CmsFileDto> CreateFileAsync(CmsFileCreateRequest request, IFor
             return CmsFileMapper.ToCmsFileDto(entity, names);
         }
 
-        private static void AssertNotStale(File entity, DateTime? lastModifiedOn)
-        {
-            if (lastModifiedOn == null)
-            {
-                throw new ArgumentException("LastModifiedOn is required so concurrent edits can be detected.");
-            }
-            // Compare to the second: serialized timestamps lose sub-second precision round-tripping
-            // through the client.
-            if (Math.Abs((entity.ModifiedOn - lastModifiedOn.Value).TotalSeconds) >= 1)
-            {
-                throw new CmsConcurrencyException(
-                    $"This file was modified by {entity.ModifiedBy} on {entity.ModifiedOn:g}. Reload to get the latest version.");
-            }
-        }
+        private static void AssertNotStale(File entity, DateTime? lastModifiedOn) =>
+            CmsServiceHelpers.AssertNotStale("file", entity.ModifiedOn, entity.ModifiedBy, lastModifiedOn);
 
         /// <summary>
         /// A replacement upload keeps the record's name and path, so its bytes must be an allowed
@@ -899,14 +887,7 @@ private static string BuildCreateDetail(File entity)
             return string.Join("; ", parts);
         }
 
-        private static List<string> CleanList(List<string> values)
-        {
-            return values
-                .Where(v => !string.IsNullOrWhiteSpace(v))
-                .Select(v => v.Trim())
-                .Distinct(StringComparer.OrdinalIgnoreCase)
-                .ToList();
-        }
+        private static List<string> CleanList(List<string> values) => CmsServiceHelpers.CleanList(values);
 
         private static string? NullIfEmpty(string? value)
         {

web/Areas/CMS/Services/CmsServiceHelpers.cs

@@ -0,0 +1,41 @@
+namespace Viper.Areas.CMS.Services
+{
+    /// <summary>
+    /// Helpers shared by the CMS write services (files, content blocks, left navs) so the
+    /// optimistic-concurrency contract and permission-list normalization cannot drift between
+    /// copies. Each service wraps these with its own entity-typed one-liner.
+    /// </summary>
+    internal static class CmsServiceHelpers
+    {
+        /// <summary>
+        /// A missing stamp is a 400 (the client must send it) and a stale one is a 409 (someone
+        /// saved after the editor loaded). Compared to the second: serialized timestamps lose
+        /// sub-second precision round-tripping through the client.
+        /// </summary>
+        public static void AssertNotStale(string noun, DateTime modifiedOn, string? modifiedBy, DateTime? lastModifiedOn)
+        {
+            if (lastModifiedOn == null)
+            {
+                throw new ArgumentException("LastModifiedOn is required so concurrent edits can be detected.");
+            }
+            if (Math.Abs((modifiedOn - lastModifiedOn.Value).TotalSeconds) >= 1)
+            {
+                throw new CmsConcurrencyException(
+                    $"This {noun} was modified by {modifiedBy} on {modifiedOn:g}. Reload to get the latest version.");
+            }
+        }
+
+        /// <summary>
+        /// Trim, drop blanks, and de-duplicate case-insensitively. A client can post
+        /// "permissions": null explicitly; treat it as empty rather than 500.
+        /// </summary>
+        public static List<string> CleanList(IEnumerable<string>? values)
+        {
+            return (values ?? [])
+                .Where(v => !string.IsNullOrWhiteSpace(v))
+                .Select(v => v.Trim())
+                .Distinct(StringComparer.OrdinalIgnoreCase)
+                .ToList();
+        }
+    }
+}