feat(gateway): add channel-backed readiness probes (#38285)

* Changelog: add channel-backed readiness probe entry

* Gateway: add channel-backed readiness probes

* Docs: describe readiness probe behavior

* Gateway: add readiness probe regression tests

* Changelog: dedupe gateway probe entries

* Docs: fix readiness startup grace description

* Changelog: remove stale readiness entry

* Gateway: cover readiness hardening

* Gateway: harden readiness probes

src/gateway/server.impl.ts

@@ -106,6 +106,7 @@ import {
   incrementPresenceVersion,
   refreshGatewayHealthSnapshot,
 } from "./server/health-state.js";
+import { createReadinessChecker } from "./server/readiness.js";
 import { loadGatewayTlsRuntime } from "./server/tls.js";
 import {
   ensureGatewayStartupAuth,
@@ -546,6 +547,17 @@ export async function startGatewayServer(
   if (cfgAtStart.gateway?.tls?.enabled && !gatewayTls.enabled) {
     throw new Error(gatewayTls.error ?? "gateway tls: failed to enable");
   }
+  const serverStartedAt = Date.now();
+  const channelManager = createChannelManager({
+    loadConfig,
+    channelLogs,
+    channelRuntimeEnvs,
+    channelRuntime: createPluginRuntime().channel,
+  });
+  const getReadiness = createReadinessChecker({
+    channelManager,
+    startedAt: serverStartedAt,
+  });
   const {
     canvasHost,
     httpServer,
@@ -589,6 +601,7 @@ export async function startGatewayServer(
     log,
     logHooks,
     logPlugins,
+    getReadiness,
   });
   let bonjourStop: (() => Promise<void>) | null = null;
   const nodeRegistry = new NodeRegistry();
@@ -618,12 +631,6 @@ export async function startGatewayServer(
   });
   let { cron, storePath: cronStorePath } = cronState;
 
-  const channelManager = createChannelManager({
-    loadConfig,
-    channelLogs,
-    channelRuntimeEnvs,
-    channelRuntime: createPluginRuntime().channel,
-  });
   const { getRuntimeSnapshot, startChannels, startChannel, stopChannel, markChannelLoggedOut } =
     channelManager;