feat(04-06): implement TemplateService and FolderStructureService

- FolderStructureService.CreateFoldersAsync creates folder hierarchy from CSV rows using BulkOperationRunner
- FolderStructureService.BuildUniquePaths deduplicates and sorts paths parent-first by slash depth
- TemplateService already committed; verified compilation and interface compliance
- FolderStructureServiceTests: 4 unit tests pass (BuildUniquePaths edge cases, deduplication, empty levels, BuildPath) + 1 skip
- TemplateServiceTests: 3 unit tests pass (interface impl, SiteTemplate defaults, SiteTemplateOptions defaults) + 2 skip

SharepointToolbox.Tests/Services/FolderStructureServiceTests.cs

@@ -0,0 +1,89 @@
+using SharepointToolbox.Core.Models;
+using SharepointToolbox.Services;
+
+namespace SharepointToolbox.Tests.Services;
+
+public class FolderStructureServiceTests
+{
+    [Fact]
+    public void FolderStructureService_Implements_IFolderStructureService()
+    {
+        Assert.True(typeof(IFolderStructureService).IsAssignableFrom(typeof(FolderStructureService)));
+    }
+
+    [Fact]
+    public void BuildUniquePaths_FromExampleCsv_ReturnsParentFirst()
+    {
+        var rows = new List<FolderStructureRow>
+        {
+            new() { Level1 = "Administration", Level2 = "Comptabilite", Level3 = "Factures" },
+            new() { Level1 = "Administration", Level2 = "Comptabilite", Level3 = "Bilans" },
+            new() { Level1 = "Administration", Level2 = "Ressources Humaines" },
+            new() { Level1 = "Projets", Level2 = "Projet Alpha", Level3 = "Documents" },
+        };
+
+        var paths = FolderStructureService.BuildUniquePaths(rows);
+
+        // Should contain unique paths, parent-first
+        Assert.Contains("Administration", paths);
+        Assert.Contains("Administration/Comptabilite", paths);
+        Assert.Contains("Administration/Comptabilite/Factures", paths);
+        Assert.Contains("Administration/Comptabilite/Bilans", paths);
+        Assert.Contains("Projets", paths);
+        Assert.Contains("Projets/Projet Alpha", paths);
+
+        // Parent-first: "Administration" before "Administration/Comptabilite"
+        var adminIdx = paths.ToList().IndexOf("Administration");
+        var compIdx = paths.ToList().IndexOf("Administration/Comptabilite");
+        Assert.True(adminIdx < compIdx);
+    }
+
+    [Fact]
+    public void BuildUniquePaths_DuplicateRows_Deduplicated()
+    {
+        var rows = new List<FolderStructureRow>
+        {
+            new() { Level1 = "A", Level2 = "B" },
+            new() { Level1 = "A", Level2 = "B" },
+            new() { Level1 = "A", Level2 = "C" },
+        };
+
+        var paths = FolderStructureService.BuildUniquePaths(rows);
+
+        Assert.Equal(3, paths.Count); // A, A/B, A/C (deduplicated)
+    }
+
+    [Fact]
+    public void BuildUniquePaths_EmptyLevels_StopsAtLastNonEmpty()
+    {
+        var rows = new List<FolderStructureRow>
+        {
+            new() { Level1 = "Root", Level2 = "", Level3 = "", Level4 = "" },
+        };
+
+        var paths = FolderStructureService.BuildUniquePaths(rows);
+
+        Assert.Single(paths);
+        Assert.Equal("Root", paths[0]);
+    }
+
+    [Fact]
+    public void FolderStructureRow_BuildPath_ReturnsCorrectPath()
+    {
+        var row = new FolderStructureRow
+        {
+            Level1 = "Admin",
+            Level2 = "HR",
+            Level3 = "Contracts",
+            Level4 = ""
+        };
+
+        Assert.Equal("Admin/HR/Contracts", row.BuildPath());
+    }
+
+    [Fact(Skip = "Requires live SharePoint tenant")]
+    public async Task CreateFoldersAsync_ValidRows_CreatesFolders()
+    {
+        await Task.CompletedTask;
+    }
+}