_adapter_enforces_own_access_policy accessed self.adapters directly, but
several auth tests build a bare GatewayRunner via object.__new__ without
setting .adapters (pitfalls.md #17). Read it defensively with getattr so a
missing/empty adapter map means "no adapter owns the policy" instead of
raising AttributeError.
Fixes 4 tests: test_feishu_bot_auth_bypass, test_discord_bot_auth_bypass (x2),
test_signal::test_signal_in_allowlist_maps.