Fix path comparison bug in UserStartNodeEntitiesService (similar to #21162)

This fixes the same path comparison bug we fixed in PR #21162 but in C# string
comparisons instead of SQL queries.

## Root Cause
Path comparisons without trailing commas caused false matches:
- Path "-1,1001" incorrectly matched prefix "-1,100"
- This marked nodes as ancestors/descendants when they weren't related

## Examples of False Matches
- child.Path = "-1,1001", startNodePath = "-1,100"
  - OLD: "-1,1001".StartsWith("-1,100") = TRUE (bug!)
  - NEW: "-1,1001,".StartsWith("-1,100,") = FALSE (correct!)

- child.Path = "-1,100", startNodePath = "-1,1001"
  - OLD: "-1,1001".StartsWith("-1,100") = TRUE (bug!)
  - NEW: "-1,1001,".StartsWith("-1,100,") = FALSE (correct!)

## Fix Applied (Two Locations)
1. Line 146 (ancestor check): Added comma suffix to child.Path
2. Line 226 (IsDescendantOrSelf): Added comma suffix to both paths

This matches the pattern already used correctly in lines 92 and 191 of the
same file, and mirrors the SQL fix from PR #21162.

## Test Impact
This should fix the failing E2E test where child media folders were incorrectly
marked as noAccess when they were actually the user's start node.

Related: #21162

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: iOvergaard

src/Umbraco.Cms.Api.Management/Services/Entities/UserStartNodeEntitiesService.cs

@@ -142,7 +142,8 @@ public IEnumerable<UserAccessEntity> ChildUserAccessEntities(IEnumerable<IEntity
             }
 
             // is ancestor of a start node?
-            if (userStartNodePaths.Any(path => path.StartsWith(child.Path)))
+            // Note: Add trailing comma to prevent false matches (e.g., path "-1,100" should not match "-1,1001")
+            if (userStartNodePaths.Any(path => path.StartsWith($"{child.Path},")))
             {
                 return new UserAccessEntity(child, false);
             }
@@ -220,5 +221,7 @@ public IEnumerable<UserAccessEntity> UserAccessEntities(IEnumerable<IEntitySlim>
         => entities.Select(entity => new UserAccessEntity(entity, IsDescendantOrSelf(entity, userStartNodePaths))).ToArray();
 
     private static bool IsDescendantOrSelf(IEntitySlim child, string[] userStartNodePaths)
-        => userStartNodePaths.Any(path => child.Path.StartsWith(path));
+        // Note: Add trailing commas to both paths to prevent false matches (e.g., path "-1,100" should not match "-1,1001")
+        // This matches the pattern used in lines 92 and 191 of this file
+        => userStartNodePaths.Any(path => $"{child.Path},".StartsWith($"{path},"));
 }