$feature_flag_called events minimized for non-experiment flags

Minor changes

• 5ef2c23 $feature_flag_called events are now minimized for non-experiment flags when the server enables it. When the /flags v2 response (minimalFlagCalledEvents) or the local-evaluation payload (minimal_flag_called_events) reports the gate as enabled and the evaluated flag has no linked experiment (has_experiment is false), the event's properties are reduced to a strict allowlist ($feature_flag, $feature_flag_response, $feature_flag_has_experiment, the $feature_flag_* debug scalars, locally_evaluated, $groups, $process_person_profile, $session_id, $lib, $lib_version, $is_server, $geoip_disable, $os, $os_version, $os_distro, $python_runtime, $python_version). Everything else — including super properties and custom event properties — is stripped from those events.

  If the server does not report the gate, if the flag's has_experiment signal is missing, or if the flag is linked to an experiment, the full property set is sent unchanged. There is no SDK-side configuration; the gate is controlled per-team by the server. For evaluate_flags() snapshots, the gate is pinned when the snapshot is created, so deferred flag accesses are shaped by the evaluation that produced them.

  Custom flag_definition_cache providers now receive an additional minimal_flag_called_events key in the definitions payload, so the gate survives external cache round-trips.

  When the server reports has_experiment for a flag, every $feature_flag_called event also carries a $feature_flag_has_experiment boolean property. — Thanks @haacked!