Adds gateway.platform_connect_timeout (default 30s) to DEFAULT_CONFIG and
bridges it to the internal HERMES_GATEWAY_PLATFORM_CONNECT_TIMEOUT env var
at gateway startup, following the existing gateway_timeout config->env
pattern. The env var remains the manual-override escape hatch and wins if
set explicitly; otherwise config.yaml supplies the value. This closes the
issue's documentation/config-surface request (#19776 suggestion 2) on top
of the adapter ready-wait fix, so users no longer need an undocumented env
var to raise the Discord connect timeout.
Refs #19776
7b12753948