Fix false negatives in integration test for unused vars

Updated tests/integration/test_vars_usage_in_yaml.py:
- Variables immediately followed by '(' are now treated as function calls,
  not as set variables. This prevents false errors.
- Fixed detection of redirect_domain_mappings so it is no longer flagged
  as unused.

See: https://chatgpt.com/share/68c3542d-f44c-800f-a483-b3e43739f315

tasks/stages/01_constructor.yml

@@ -61,6 +61,7 @@
           canonical_domains_map(PRIMARY_DOMAIN) | 
           combine(CURRENT_PLAY_DOMAINS, recursive=True) 
         }}
+
   - name: Merge redirect_domain_mappings
     set_fact:
       # The following mapping is necessary to define the exceptions for domains which are created, but which aren't used