fix(ocpp-server): share charge_points set across connections and harden test quality

fix(ocpp-server): share charge_points set across connections and harden test quality

- Fix charge_points set created per-connection instead of shared across
  all connections via ServerConfig
- Remove AuthConfig.__getitem__ dict-compat hack and isinstance(dict)
  coercion branch from ChargePoint.__init__
- Migrate all test fixtures from raw dicts to AuthConfig instances
- Replace 11 weak 'assert X is not None' with typed assertions
  (isinstance, enum equality)
- Hoist Action import to module level in test_server.py