feat(post_creation): added multiple image posts creation and validation

lib/features/blog_post_create/validation/blog_post_form_validator.dart

@@ -0,0 +1,158 @@
+/// Client-side validation aligned with backend BlogPost V2 create/update rules.
+abstract final class BlogPostFormValidator {
+  static const minTextLength = 3;
+  static const maxVideoUrlLength = 2048;
+  static const maxGalleryImages = 10;
+  static const allowedImageExtensions = {'jpg', 'jpeg', 'png'};
+
+  static const titleTooShort = 'Заголовок должен содержать минимум 3 символа';
+  static const descriptionTooShort =
+      'Описание должно содержать минимум 3 символа';
+  static const contentRequired =
+      'Добавьте заголовок, описание, изображение или ссылку на видео';
+  static const galleryTooLarge = 'Можно загрузить не более 10 изображений';
+  static const imageTypeInvalid = 'Допустимы только изображения JPG или PNG';
+  static const videoUrlTooLong = 'Ссылка на видео слишком длинная';
+  static const videoUrlInvalid = 'Укажите корректную ссылку на видео';
+  static const videoUrlUnsupportedHost =
+      'Поддерживаются только Rutube, VK и OK';
+  static const videoUrlRutubeFormat = 'Неверный формат ссылки Rutube';
+  static const videoUrlVkFormat = 'Неверный формат ссылки VK';
+  static const videoUrlOkFormat = 'Неверный формат ссылки OK';
+
+  /// Empty is allowed; non-empty must be at least [minTextLength] chars (after trim).
+  static String? title(String? value) {
+    final text = value?.trim() ?? '';
+    if (text.isEmpty) {
+      return null;
+    }
+    if (text.length < minTextLength) {
+      return titleTooShort;
+    }
+    return null;
+  }
+
+  /// Empty is allowed; non-empty must be at least [minTextLength] chars (after trim).
+  static String? description(String? value) {
+    final text = value?.trim() ?? '';
+    if (text.isEmpty) {
+      return null;
+    }
+    if (text.length < minTextLength) {
+      return descriptionTooShort;
+    }
+    return null;
+  }
+
+  /// Empty is allowed; non-empty must match allowed video hosts/paths.
+  static String? videoUrl(String? value) {
+    final url = value?.trim() ?? '';
+    if (url.isEmpty) {
+      return null;
+    }
+    if (url.length > maxVideoUrlLength) {
+      return videoUrlTooLong;
+    }
+
+    final uri = Uri.tryParse(url);
+    if (uri == null || !uri.hasScheme || uri.host.isEmpty) {
+      return videoUrlInvalid;
+    }
+    if (uri.scheme != 'http' && uri.scheme != 'https') {
+      return videoUrlInvalid;
+    }
+
+    final host = uri.host.toLowerCase();
+    if (!_hostMatchesRootAllowlist(host, const [
+      'vk.com',
+      'vkvideo.ru',
+      'ok.ru',
+      'rutube.ru',
+    ])) {
+      return videoUrlUnsupportedHost;
+    }
+
+    final path = uri.path;
+    if (_hostMatchesRootAllowlist(host, const ['rutube.ru'])) {
+      if (!_rutubePath.hasMatch(path)) {
+        return videoUrlRutubeFormat;
+      }
+      return null;
+    }
+    if (_hostMatchesRootAllowlist(host, const ['vk.com', 'vkvideo.ru'])) {
+      if (!_vkPath.hasMatch(path)) {
+        return videoUrlVkFormat;
+      }
+      return null;
+    }
+    if (_hostMatchesRootAllowlist(host, const ['ok.ru'])) {
+      if (!_okPath.hasMatch(path)) {
+        return videoUrlOkFormat;
+      }
+    }
+    return null;
+  }
+
+  /// At least one of title / description / images / video must be meaningfully filled.
+  static String? contentRequiredMessage({
+    required String title,
+    required String description,
+    required String videoUrl,
+    required int imageCount,
+  }) {
+    final hasTitle = title.trim().length >= minTextLength;
+    final hasDescription = description.trim().length >= minTextLength;
+    final trimmedVideo = videoUrl.trim();
+    final hasVideo =
+        trimmedVideo.isNotEmpty &&
+        BlogPostFormValidator.videoUrl(trimmedVideo) == null;
+    final hasImages = imageCount > 0;
+
+    if (hasTitle || hasDescription || hasVideo || hasImages) {
+      return null;
+    }
+    return contentRequired;
+  }
+
+  static String? galleryCount(int count) {
+    if (count > maxGalleryImages) {
+      return galleryTooLarge;
+    }
+    return null;
+  }
+
+  /// Returns error if any path has a non-allowed extension. Empty path is ignored.
+  static String? imagePaths(Iterable<String> paths) {
+    for (final path in paths) {
+      final name = path.split(RegExp(r'[\\/]')).last;
+      final dot = name.lastIndexOf('.');
+      if (dot < 0 || dot == name.length - 1) {
+        return imageTypeInvalid;
+      }
+      final ext = name.substring(dot + 1).toLowerCase();
+      if (!allowedImageExtensions.contains(ext)) {
+        return imageTypeInvalid;
+      }
+    }
+    return null;
+  }
+
+  static final _rutubePath = RegExp(
+    r'^/(video|shorts)/[a-z0-9]+/?$',
+    caseSensitive: false,
+  );
+  static final _vkPath = RegExp(r'^/video-?\d+_\d+/?$', caseSensitive: false);
+  static final _okPath = RegExp(r'^/video/\d+/?$');
+
+  static bool _hostMatchesRootAllowlist(
+    String host,
+    List<String> allowedRootHosts,
+  ) {
+    for (final allowed in allowedRootHosts) {
+      if (host == allowed || host.endsWith('.$allowed')) {
+        return true;
+      }
+    }
+    return false;
+  }
+}