fix(gateway): hot-reload channelHealthCheckMinutes without full restart

The health monitor was created once at startup and never touched by
applyHotReload(), so changing channelHealthCheckMinutes only took
effect after a full gateway restart.

Wire up a "restart-health-monitor" reload action so hot-reload can
stop the old monitor and (re)create one with the updated interval —
or disable it entirely when set to 0.

Closes #32105

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/gateway/server.impl.ts

@@ -656,7 +656,7 @@ export async function startGatewayServer(
 
   const healthCheckMinutes = cfgAtStart.gateway?.channelHealthCheckMinutes;
   const healthCheckDisabled = healthCheckMinutes === 0;
-  const channelHealthMonitor = healthCheckDisabled
+  let channelHealthMonitor = healthCheckDisabled
     ? null
     : startChannelHealthMonitor({
         channelManager,
@@ -841,6 +841,7 @@ export async function startGatewayServer(
             heartbeatRunner,
             cronState,
             browserControl,
+            channelHealthMonitor,
           }),
           setState: (nextState) => {
             hooksConfig = nextState.hooksConfig;
@@ -849,6 +850,7 @@ export async function startGatewayServer(
             cron = cronState.cron;
             cronStorePath = cronState.storePath;
             browserControl = nextState.browserControl;
+            channelHealthMonitor = nextState.channelHealthMonitor;
           },
           startChannel,
           stopChannel,
@@ -857,6 +859,8 @@ export async function startGatewayServer(
           logChannels,
           logCron,
           logReload,
+          createHealthMonitor: (checkIntervalMs: number) =>
+            startChannelHealthMonitor({ channelManager, checkIntervalMs }),
         });
 
         return startGatewayConfigReloader({