chore: ensure agent conn routine is closed before exit (#6900)

This caused a leak in `main`!

codersdk/agentsdk/agentsdk.go

@@ -161,12 +161,15 @@ func (c *Client) Listen(ctx context.Context) (net.Conn, error) {
 		return nil, codersdk.ReadBodyAsError(res)
 	}
 
+	ctx, cancelFunc := context.WithCancel(ctx)
 	ctx, wsNetConn := websocketNetConn(ctx, conn, websocket.MessageBinary)
 
 	// Ping once every 30 seconds to ensure that the websocket is alive. If we
 	// don't get a response within 30s we kill the websocket and reconnect.
 	// See: https://github.com/coder/coder/pull/5824
+	closed := make(chan struct{})
 	go func() {
+		defer close(closed)
 		tick := 30 * time.Second
 		ticker := time.NewTicker(tick)
 		defer ticker.Stop()
@@ -199,7 +202,13 @@ func (c *Client) Listen(ctx context.Context) (net.Conn, error) {
 		}
 	}()
 
-	return wsNetConn, nil
+	return &closeNetConn{
+		Conn: wsNetConn,
+		closeFunc: func() {
+			cancelFunc()
+			<-closed
+		},
+	}, nil
 }
 
 type PostAppHealthsRequest struct {
@@ -623,3 +632,13 @@ type StartupLogsNotifyMessage struct {
 	CreatedAfter int64 `json:"created_after"`
 	EndOfLogs    bool  `json:"end_of_logs"`
 }
+
+type closeNetConn struct {
+	net.Conn
+	closeFunc func()
+}
+
+func (c *closeNetConn) Close() error {
+	c.closeFunc()
+	return c.Conn.Close()
+}