[AI-FSSDK] [FSSDK-12337] Fix test structure, mypy and ruff compliance

- Move feature rollout tests from standalone test_feature_rollout.py into
  test_config.py following module-level testing convention
- Use base.BaseTest instead of unittest.TestCase for consistency
- Fix mypy strict type errors in Variation construction using cast
- All checks pass: ruff, mypy --strict, pytest (941/941)

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

Author: FarhanAnjum-opti

optimizely/project_config.py

@@ -251,7 +251,15 @@ def __init__(self, datafile: str | bytes, logger: Logger, error_handler: Any):
                                 'endOfRange': 10000,
                             })
                             # Update variation maps for this experiment
-                            var_entity = entities.Variation(**everyone_else_variation)
+                            var_entity = entities.Variation(
+                                id=everyone_else_variation['id'],
+                                key=everyone_else_variation['key'],
+                                featureEnabled=bool(everyone_else_variation.get('featureEnabled', False)),
+                                variables=cast(
+                                    Optional[list[entities.Variable]],
+                                    everyone_else_variation.get('variables'),
+                                ),
+                            )
                             self.variation_key_map[experiment.key][var_entity.key] = var_entity
                             self.variation_id_map[experiment.key][var_entity.id] = var_entity
                             self.variation_id_map_by_experiment_id[experiment.id][var_entity.id] = var_entity