fix(instance): resolve empty pairing code on POST /instance/pair

The Pair function was calling PairPhone directly without checking if
the client was initialized, and was silently swallowing errors from
PairPhone. This caused two problems:

1. If the client was nil or disconnected, PairPhone returned an error
   but the function ignored it and returned HTTP 200 with an empty
   PairingCode field, misleading the caller into thinking it succeeded.

2. The client was never started before PairPhone was called. WhatsApp
   requires the client to be connected to the WA websocket (waiting
   for auth) before a pairing code can be generated.

Fix:
- Start the instance automatically if no active connection exists,
  mirroring the QR code flow
- Wait 3 seconds for the WA websocket connection to establish and
  the initial QR generation to begin (required by whatsmeow before
  PairPhone can be called)
- Reject early if the instance is already authenticated
- Return PairPhone errors to the caller instead of swallowing them,
  so the handler correctly responds with HTTP 500 and an actionable
  error message

Fixes #21

Author: edilsonoliveirama

pkg/instance/service/instance_service.go

@@ -471,9 +471,35 @@ func (i instances) GetQr(instance *instance_model.Instance) (*QrcodeStruct, erro
 }
 
 func (i instances) Pair(data *PairStruct, instance *instance_model.Instance) (*PairReturnStruct, error) {
-	code, err := i.clientPointer[instance.Id].PairPhone(context.Background(), data.Phone, true, whatsmeow.PairClientChrome, "Chrome (Linux)")
+	logger := i.loggerWrapper.GetLogger(instance.Id)
+	client := i.clientPointer[instance.Id]
+
+	if client == nil || !client.IsConnected() {
+		if client != nil && client.IsLoggedIn() {
+			return nil, fmt.Errorf("instance is already authenticated")
+		}
+		logger.LogInfo("[%s] No active connection, starting instance for phone pairing", instance.Id)
+		if err := i.whatsmeowService.StartInstance(instance.Id); err != nil {
+			logger.LogError("[%s] Failed to start instance for pairing: %v", instance.Id, err)
+			return nil, fmt.Errorf("failed to start instance: %w", err)
+		}
+		// Wait for the WA websocket connection and initial QR generation to establish.
+		// PairPhone must be called after the QR event is received per whatsmeow docs.
+		time.Sleep(3 * time.Second)
+		client = i.clientPointer[instance.Id]
+		if client == nil {
+			return nil, fmt.Errorf("failed to initialize client for pairing")
+		}
+	}
+
+	if client.IsLoggedIn() {
+		return nil, fmt.Errorf("instance is already authenticated")
+	}
+
+	code, err := client.PairPhone(context.Background(), data.Phone, true, whatsmeow.PairClientChrome, "Chrome (Linux)")
 	if err != nil {
-		i.loggerWrapper.GetLogger(instance.Id).LogError("[%s] something went wrong calling pair phone", instance.Id)
+		logger.LogError("[%s] PairPhone failed: %v", instance.Id, err)
+		return nil, fmt.Errorf("pairing failed: %w", err)
 	}
 
 	return &PairReturnStruct{PairingCode: code}, nil