fix(coderd): ensure agent WebSocket conn is cleaned up (#19711)

When clients disconnected from the /containers/watch endpoint, the WebSocket 
connection between coderd and the agent stayed open. This caused heartbeat 
traffic every 15s that was incorrectly counted as workspace activity, 
extending workspace lifetimes indefinitely.

Now properly cancels the agent connection context when the client disconnects.

coderd/workspaceagents.go

@@ -817,12 +817,13 @@ func (api *API) watchWorkspaceAgentContainers(rw http.ResponseWriter, r *http.Re
 	var (
 		ctx            = r.Context()
 		workspaceAgent = httpmw.WorkspaceAgentParam(r)
+		logger         = api.Logger.Named("agent_container_watcher").With(slog.F("agent_id", workspaceAgent.ID))
 	)
 
 	// If the agent is unreachable, the request will hang. Assume that if we
 	// don't get a response after 30s that the agent is unreachable.
-	dialCtx, cancel := context.WithTimeout(ctx, 30*time.Second)
-	defer cancel()
+	dialCtx, dialCancel := context.WithTimeout(ctx, 30*time.Second)
+	defer dialCancel()
 	apiAgent, err := db2sdk.WorkspaceAgent(
 		api.DERPMap(),
 		*api.TailnetCoordinator.Load(),
@@ -857,8 +858,7 @@ func (api *API) watchWorkspaceAgentContainers(rw http.ResponseWriter, r *http.Re
 	}
 	defer release()
 
-	watcherLogger := api.Logger.Named("agent_container_watcher").With(slog.F("agent_id", workspaceAgent.ID))
-	containersCh, closer, err := agentConn.WatchContainers(ctx, watcherLogger)
+	containersCh, closer, err := agentConn.WatchContainers(ctx, logger)
 	if err != nil {
 		httpapi.Write(ctx, rw, http.StatusInternalServerError, codersdk.Response{
 			Message: "Internal error watching agent's containers.",
@@ -877,6 +877,9 @@ func (api *API) watchWorkspaceAgentContainers(rw http.ResponseWriter, r *http.Re
 		return
 	}
 
+	ctx, cancel := context.WithCancel(r.Context())
+	defer cancel()
+
 	// Here we close the websocket for reading, so that the websocket library will handle pings and
 	// close frames.
 	_ = conn.CloseRead(context.Background())
@@ -884,7 +887,7 @@ func (api *API) watchWorkspaceAgentContainers(rw http.ResponseWriter, r *http.Re
 	ctx, wsNetConn := codersdk.WebsocketNetConn(ctx, conn, websocket.MessageText)
 	defer wsNetConn.Close()
 
-	go httpapi.Heartbeat(ctx, conn)
+	go httpapi.HeartbeatClose(ctx, logger, cancel, conn)
 
 	encoder := json.NewEncoder(wsNetConn)