fix(seer): Fix optimistic cache update spreading wrong object into preference field

The onSuccess handler was spreading the entire SeerPreferencesResponse object
(which contains code_mapping_repos and preference) directly into the preference
field. The preference field expects ProjectSeerPreferences (repositories,
automated_run_stopping_point, automation_handoff), so this created a malformed
cache entry with a nested preference key and a misplaced code_mapping_repos field.

Fix by spreading preferencePayload.preference instead of preferencePayload.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: ryan953

static/app/views/settings/seer/overview/utils/seerStoppingPoint.ts

@@ -191,7 +191,7 @@ export function getProjectStoppingPointMutationOptions({
             preference: {
               repositories: [],
               ...previous.preference,
-              ...preferencePayload,
+              ...preferencePayload.preference,
             },
           });
         }