Phase 2 task 2.4 (the script part). The script enforces:
PROVIDERS is declared as a literal only in src/ai_client.py.
The __getattr__ re-export in src/models.py is allowed (it
lazy-imports, not a literal declaration).
Catches the literal pattern 'PROVIDERS: List[str] = ['
specifically, which the __getattr__ re-export does not
match.
OK: passes against current state where PROVIDERS is
declared only in src/ai_client.py:56.
