Extract shared suggestion validation and remove dead DTO

Endpoints/SuggestionValidator.cs

@@ -0,0 +1,46 @@
+using GameList.Contracts;
+
+namespace GameList.Endpoints;
+
+internal static class SuggestionValidator
+{
+    public static async Task<string?> ValidateAsync(SuggestionRequest request, IHttpClientFactory httpFactory)
+    {
+        if (string.IsNullOrWhiteSpace(request.Name) || request.Name.Length > 100)
+            return "Name is required and must be <= 100 characters.";
+
+        if (!EndpointHelpers.IsValidImageUrl(request.ScreenshotUrl))
+            return "Screenshot URL must be http(s) and end with an image file extension.";
+
+        if (!await EndpointHelpers.IsReachableImageAsync(request.ScreenshotUrl, httpFactory))
+            return "Screenshot URL could not be validated as an image. Use a public image link (http/https, no redirects, max 5 MB).";
+
+        if (!EndpointHelpers.IsValidHttpUrl(request.GameUrl))
+            return "Game URL must be http or https.";
+
+        if (!EndpointHelpers.IsValidHttpUrl(request.YoutubeUrl))
+            return "YouTube URL must be http or https.";
+
+        return ValidatePlayers(request.MinPlayers, request.MaxPlayers);
+    }
+
+    private static string? ValidatePlayers(int? minPlayers, int? maxPlayers)
+    {
+        if (minPlayers is null && maxPlayers is null)
+            return null;
+
+        if (minPlayers is < 1 or > 32)
+            return "Min players must be between 1 and 32.";
+
+        if (maxPlayers is < 1 or > 32)
+            return "Max players must be between 1 and 32.";
+
+        if (minPlayers is null || maxPlayers is null)
+            return "Provide both min and max players.";
+
+        if (minPlayers > maxPlayers)
+            return "Min players cannot exceed max players.";
+
+        return null;
+    }
+}