fix: stop reading closed channel for /watch devcontainers endpoint (coder#19373) (coder#20095)

Fixes coder#19372

Author: DanielleMaywood

agent/agentcontainers/api.go

@@ -763,7 +763,11 @@ func (api *API) broadcastUpdatesLocked() {
 func (api *API) watchContainers(rw http.ResponseWriter, r *http.Request) {
 	ctx := r.Context()
 
-	conn, err := websocket.Accept(rw, r, nil)
+	conn, err := websocket.Accept(rw, r, &websocket.AcceptOptions{
+		// We want `NoContextTakeover` compression to balance improving
+		// bandwidth cost/latency with minimal memory usage overhead.
+		CompressionMode: websocket.CompressionNoContextTakeover,
+	})
 	if err != nil {
 		httpapi.Write(ctx, rw, http.StatusInternalServerError, codersdk.Response{
 			Message: "Failed to upgrade connection to websocket.",

coderd/workspaceagents.go

@@ -899,7 +899,11 @@ func (api *API) watchWorkspaceAgentContainers(rw http.ResponseWriter, r *http.Re
 		case <-ctx.Done():
 			return
 
-		case containers := <-containersCh:
+		case containers, ok := <-containersCh:
+			if !ok {
+				return
+			}
+
 			if err := encoder.Encode(containers); err != nil {
 				api.Logger.Error(ctx, "encode containers", slog.Error(err))
 				return