refactor: migrate ENABLE_EXPORT_GIT test override off FEATURES-as-dict

The production reader is a SettingToggle('ENABLE_EXPORT_GIT') that reads the flat
settings.ENABLE_EXPORT_GIT. The git-export command test still enabled it by building a
FEATURES_WITH_EXPORT_GIT = settings.FEATURES.copy() and applying
@override_settings(FEATURES=...). Drop the dict copy and fold ENABLE_EXPORT_GIT=True into
the class-level override_settings so no settings.FEATURES usage remains; the SettingToggle
picks up the flat override.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>