Two related bugs in gateway/config.py prevented per-platform
gateway_restart_notification from working through config.yaml:
1. The shared-key bridging loop (load_gateway_config) omitted
'gateway_restart_notification', so the key never landed in
platform_data['extra'] even when set under e.g. 'discord:' or
'mattermost:' sections.
2. PlatformConfig.from_dict() only read gateway_restart_notification
from the top-level data dict, ignoring the 'extra' sub-dict where
bridged keys are stored.
Fix: add the key to the bridging loop, and add an 'extra' fallback
in from_dict() so that round-tripped values (YAML → bridged → extra
→ from_dict) resolve correctly.
Impact: users can now set gateway_restart_notification: false per
platform in config.yaml instead of relying on env vars or the
global platforms: block.
7a46c68857