Complete the YAML null handling for all three SessionResetPolicy fields.
at_hour and idle_minutes already had null coalescing; mode was still
using data.get('mode', 'both') which returns None when the key exists
with an explicit null value.
Add regression test covering all-null input.
Based on PR #1120 by stablegenius49.
Follow-up on salvaged PR #975.
Bridge quick_commands from config.yaml into load_gateway_config(),
normalize non-dict quick command config at runtime, and add coverage
for GatewayConfig round-trips plus config.yaml bridging. This makes the
GatewayConfig quick-command fix complete for the real user-facing config
path implicated by issue #973.
