The shared-key bridging loop (allow_from, require_mention,
free_response_channels, …) read only the top-level yaml platform block
(yaml_cfg.get(plat.value)). When a user configured a platform solely
under ``platforms:`` or ``gateway.platforms:`` with no top-level block,
the loop skipped that platform entirely and all bridged keys were silently
dropped into PlatformConfig.extra — making allow_from, require_mention,
etc. ineffective for nested-only configs.
The apply_yaml_config_fn dispatch already received this same fallback in
44f3e51 to handle plugin adapters (e.g. Discord allow_from). The
shared-key loop now mirrors it: if yaml_cfg.get(plat.value) is absent,
fall back to gateway.platforms.<name> then platforms.<name>.
The enabled field is deliberately excluded from the nested fallback
(guarded by _cfg_toplevel): _merge_platform_map already merged it with
the correct precedence, so re-applying it from a single nested source
would overwrite the correctly-merged value.
Two new regression tests assert that allow_from and require_mention
configured under platforms.telegram and gateway.platforms.telegram are
bridged into PlatformConfig.extra. All 54 existing config tests pass.
4ae3c988b5