Fix registry search 500: handle non-JSON Harbor fixture response

Harbor fixture returns plain text, not JSON. The RegistrySearchEndpoints
deserialization crashed with an unhandled JsonException causing 500 on
every /api/v1/registries/images/search request. This blocked the release
creation wizard at Step 2 (Components).

Fix: catch JsonException and return empty results gracefully. The release
wizard now shows "no results" instead of silently failing.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/Platform/StellaOps.Platform.WebService/Endpoints/RegistrySearchEndpoints.cs

@@ -109,6 +109,17 @@ public static class RegistrySearchEndpoints
                 RegistryId = "harbor-fixture"
             });
         }
+        catch (JsonException jex)
+        {
+            logger.LogWarning(jex, "Harbor returned non-JSON response for query '{Query}'", query);
+
+            return Results.Ok(new RegistrySearchResponse
+            {
+                Items = [],
+                TotalCount = 0,
+                RegistryId = "harbor-fixture"
+            });
+        }
         catch (Exception ex) when (ex is HttpRequestException or TaskCanceledException or OperationCanceledException)
         {
             logger.LogWarning(ex, "Harbor registry unreachable during image search for query '{Query}'", query);