feat: bump workspace last_used_at on chat heartbeat (#23205)

- coderd: Wires `options.WorkspaceUsageTracker` into the chatd config.
- chatd: Adds `UsageTracker` and calls `UsageTracker.Add(workspaceID)`
on each heartbeat tick
- chatd: adds tests to verify `last_used_at` bump behaviour

> 🤖 This PR was created with the help of Coder Agents, and will be
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coderd/coderd.go

@@ -787,6 +787,7 @@ func New(options *Options) *API {
 		StartWorkspace:     api.chatStartWorkspace,
 		Pubsub:             options.Pubsub,
 		WebpushDispatcher:  options.WebPushDispatcher,
+		UsageTracker:       options.WorkspaceUsageTracker,
 	})
 	gitSyncLogger := options.Logger.Named("gitsync")
 	refresher := gitsync.NewRefresher(