The gateway config loader read config.yaml but never merged its
`platforms` key into the runtime config dict. This meant that
platform-specific settings defined under `platforms.<name>.extra`
(e.g. webhook routes) were silently ignored unless the user also
duplicated them in the legacy gateway.json file.
Merge `yaml_cfg["platforms"]` into `gw_data["platforms"]` with a
shallow deep-merge of the `extra` dict so that gateway.json defaults
are preserved while config.yaml values take precedence.
Closes#2305
1830db0476