Fix remote image language priority to prefer English over no-language

Previously, images with no language were ranked higher (score 3) than
English images (score 2), causing poorly rated languageless images to
be selected over well-rated English alternatives for posters and logos.

Swap the priority so English is preferred over no-language images.
Backdrop images are unaffected as they have their own dedicated sorting.

Add unit tests for OrderByLanguageDescending.

Fixes #13310

tests/Jellyfin.Model.Tests/Extensions/EnumerableExtensionsTests.cs

@@ -0,0 +1,117 @@
+using System.Linq;
+using MediaBrowser.Model.Extensions;
+using MediaBrowser.Model.Providers;
+using Xunit;
+
+namespace Jellyfin.Model.Tests.Extensions
+{
+    public class EnumerableExtensionsTests
+    {
+        [Fact]
+        public void OrderByLanguageDescending_PreferredLanguageFirst()
+        {
+            var images = new[]
+            {
+                new RemoteImageInfo { Language = "en", CommunityRating = 5.0, VoteCount = 100 },
+                new RemoteImageInfo { Language = "de", CommunityRating = 9.0, VoteCount = 200 },
+                new RemoteImageInfo { Language = null, CommunityRating = 7.0, VoteCount = 50 },
+                new RemoteImageInfo { Language = "fr", CommunityRating = 8.0, VoteCount = 150 },
+            };
+
+            var result = images.OrderByLanguageDescending("de").ToList();
+
+            Assert.Equal("de", result[0].Language);
+            Assert.Equal("en", result[1].Language);
+            Assert.Null(result[2].Language);
+            Assert.Equal("fr", result[3].Language);
+        }
+
+        [Fact]
+        public void OrderByLanguageDescending_EnglishBeforeNoLanguage()
+        {
+            var images = new[]
+            {
+                new RemoteImageInfo { Language = null, CommunityRating = 9.0, VoteCount = 500 },
+                new RemoteImageInfo { Language = "en", CommunityRating = 3.0, VoteCount = 10 },
+            };
+
+            var result = images.OrderByLanguageDescending("de").ToList();
+
+            // English should come before no-language, even with lower rating
+            Assert.Equal("en", result[0].Language);
+            Assert.Null(result[1].Language);
+        }
+
+        [Fact]
+        public void OrderByLanguageDescending_SameLanguageSortedByRatingThenVoteCount()
+        {
+            var images = new[]
+            {
+                new RemoteImageInfo { Language = "de", CommunityRating = 5.0, VoteCount = 100 },
+                new RemoteImageInfo { Language = "de", CommunityRating = 9.0, VoteCount = 50 },
+                new RemoteImageInfo { Language = "de", CommunityRating = 9.0, VoteCount = 200 },
+            };
+
+            var result = images.OrderByLanguageDescending("de").ToList();
+
+            Assert.Equal(200, result[0].VoteCount);
+            Assert.Equal(50, result[1].VoteCount);
+            Assert.Equal(100, result[2].VoteCount);
+        }
+
+        [Fact]
+        public void OrderByLanguageDescending_NullRequestedLanguage_DefaultsToEnglish()
+        {
+            var images = new[]
+            {
+                new RemoteImageInfo { Language = "fr", CommunityRating = 9.0, VoteCount = 500 },
+                new RemoteImageInfo { Language = "en", CommunityRating = 5.0, VoteCount = 10 },
+            };
+
+            var result = images.OrderByLanguageDescending(null!).ToList();
+
+            // With null requested language, English becomes the preferred language (score 4)
+            Assert.Equal("en", result[0].Language);
+            Assert.Equal("fr", result[1].Language);
+        }
+
+        [Fact]
+        public void OrderByLanguageDescending_EnglishRequested_NoDoubleBoost()
+        {
+            // When requested language IS English, "en" gets score 4 (requested match),
+            // no-language gets score 2, others get score 0
+            var images = new[]
+            {
+                new RemoteImageInfo { Language = null, CommunityRating = 9.0, VoteCount = 500 },
+                new RemoteImageInfo { Language = "en", CommunityRating = 3.0, VoteCount = 10 },
+                new RemoteImageInfo { Language = "fr", CommunityRating = 8.0, VoteCount = 300 },
+            };
+
+            var result = images.OrderByLanguageDescending("en").ToList();
+
+            Assert.Equal("en", result[0].Language);
+            Assert.Null(result[1].Language);
+            Assert.Equal("fr", result[2].Language);
+        }
+
+        [Fact]
+        public void OrderByLanguageDescending_FullPriorityOrder()
+        {
+            var images = new[]
+            {
+                new RemoteImageInfo { Language = "fr", CommunityRating = 9.0, VoteCount = 500 },
+                new RemoteImageInfo { Language = null, CommunityRating = 8.0, VoteCount = 400 },
+                new RemoteImageInfo { Language = "en", CommunityRating = 7.0, VoteCount = 300 },
+                new RemoteImageInfo { Language = "de", CommunityRating = 6.0, VoteCount = 200 },
+            };
+
+            var result = images.OrderByLanguageDescending("de").ToList();
+
+            // Expected order: de (requested) > en > no-language > fr (other)
+            Assert.Equal("de", result[0].Language);
+            Assert.Equal("en", result[1].Language);
+            Assert.Null(result[2].Language);
+            Assert.Equal("fr", result[3].Language);
+        }
+    }
+}