refactor: implement dry run feature to validate connections and count source rows without migrating

internal/app/azure/main.go

@@ -70,6 +70,15 @@ func (c *Client) UploadBuffer(ctx context.Context, containerName, blobPath strin
 	return nil
 }
 
+func (c *Client) Ping(ctx context.Context) error {
+	pager := c.client.NewListBlobsFlatPager(c.azureStorageConfig.Container, nil)
+	_, err := pager.NextPage(ctx)
+	if err != nil {
+		return fmt.Errorf("storage access check failed: %w", err)
+	}
+	return nil
+}
+
 func (c *Client) UploadAndGetURL(ctx context.Context, blobPath string, buffer []byte) (string, error) {
 	if blobPath == "" || buffer == nil {
 		return "", ErrInvalidInput