Honor episode NFO season during metadata merge

Path-derived season numbers could win over explicit provider metadata during episode refresh, causing episodes to appear in the wrong season. Prefer provider-supplied season numbers in the provider merge phases without letting later backfill merges clobber them.

tests/Jellyfin.Providers.Tests/TV/EpisodeMetadataServiceTests.cs

@@ -0,0 +1,110 @@
+using System;
+using MediaBrowser.Controller.Configuration;
+using MediaBrowser.Controller.Entities.TV;
+using MediaBrowser.Controller.IO;
+using MediaBrowser.Controller.Library;
+using MediaBrowser.Controller.Persistence;
+using MediaBrowser.Controller.Providers;
+using MediaBrowser.Model.Entities;
+using MediaBrowser.Model.IO;
+using MediaBrowser.Providers.Manager;
+using MediaBrowser.Providers.TV;
+using Microsoft.Extensions.Logging.Abstractions;
+using Moq;
+using Xunit;
+
+namespace Jellyfin.Providers.Tests.TV;
+
+public class EpisodeMetadataServiceTests
+{
+    private readonly TestEpisodeMetadataService _service = new();
+
+    [Fact]
+    public void MergeData_ProviderSeasonOverridesPathDerivedSeason()
+    {
+        var source = new MetadataResult<Episode>
+        {
+            Item = new Episode
+            {
+                ParentIndexNumber = 2
+            }
+        };
+
+        var target = new MetadataResult<Episode>
+        {
+            Item = new Episode
+            {
+                ParentIndexNumber = 1
+            }
+        };
+
+        _service.Merge(source, target, replaceData: false, mergeMetadataSettings: true);
+
+        Assert.Equal(2, target.Item.ParentIndexNumber);
+    }
+
+    [Fact]
+    public void MergeData_BackfillExistingMetadata_DoesNotOverrideProviderSeason()
+    {
+        var existingMetadata = new MetadataResult<Episode>
+        {
+            Item = new Episode
+            {
+                ParentIndexNumber = 1
+            }
+        };
+
+        var temp = new MetadataResult<Episode>
+        {
+            Item = new Episode
+            {
+                ParentIndexNumber = 2
+            }
+        };
+
+        _service.Merge(existingMetadata, temp, replaceData: false, mergeMetadataSettings: false);
+
+        Assert.Equal(2, temp.Item.ParentIndexNumber);
+    }
+
+    [Fact]
+    public void MergeData_MissingProviderSeasonKeepsExistingSeason()
+    {
+        var source = new MetadataResult<Episode>
+        {
+            Item = new Episode()
+        };
+
+        var target = new MetadataResult<Episode>
+        {
+            Item = new Episode
+            {
+                ParentIndexNumber = 1
+            }
+        };
+
+        _service.Merge(source, target, replaceData: false, mergeMetadataSettings: true);
+
+        Assert.Equal(1, target.Item.ParentIndexNumber);
+    }
+
+    private sealed class TestEpisodeMetadataService : EpisodeMetadataService
+    {
+        public TestEpisodeMetadataService()
+            : base(
+                Mock.Of<IServerConfigurationManager>(),
+                NullLogger<EpisodeMetadataService>.Instance,
+                Mock.Of<IProviderManager>(),
+                Mock.Of<IFileSystem>(),
+                Mock.Of<ILibraryManager>(),
+                Mock.Of<IExternalDataManager>(),
+                Mock.Of<IItemRepository>())
+        {
+        }
+
+        public void Merge(MetadataResult<Episode> source, MetadataResult<Episode> target, bool replaceData, bool mergeMetadataSettings)
+        {
+            MergeData(source, target, Array.Empty<MetadataField>(), replaceData, mergeMetadataSettings);
+        }
+    }
+}