feat(11-02): extend export tests to verify branding injection across all 5 services

- HtmlExportServiceTests: 3 new tests (MSP logo only, null branding no img, both logos)
- SearchExportServiceTests: 1 new branding test (img tag present when branding provided)
- StorageHtmlExportServiceTests: 1 new branding test (img tag present)
- DuplicatesHtmlExportServiceTests: 1 new branding test (img tag present)
- UserAccessHtmlExportServiceTests: 1 new branding test (img tag present)
- MakeBranding helper added to each test class
- All 45 export tests pass; full suite 247/247 with 0 failures

SharepointToolbox.Tests/Services/Export/UserAccessHtmlExportServiceTests.cs

@@ -13,6 +13,13 @@ public class UserAccessHtmlExportServiceTests
 {
     // ── Helper factory ────────────────────────────────────────────────────────
 
+    private static ReportBranding MakeBranding(bool msp = true, bool client = false)
+    {
+        var mspLogo    = msp    ? new LogoData { Base64 = "bXNw", MimeType = "image/png" }  : null;
+        var clientLogo = client ? new LogoData { Base64 = "Y2xpZW50", MimeType = "image/jpeg" } : null;
+        return new ReportBranding(mspLogo, clientLogo);
+    }
+
     private static UserAccessEntry MakeEntry(
         string userDisplay = "Alice Smith",
         string userLogin = "alice@contoso.com",
@@ -124,4 +131,14 @@ public class UserAccessHtmlExportServiceTests
         // Encoded form must be present
         Assert.Contains("<script>", html);
     }
+
+    // ── Branding tests ────────────────────────────────────────────────────────
+
+    [Fact]
+    public void BuildHtml_WithBranding_ContainsLogoImg()
+    {
+        var svc = new UserAccessHtmlExportService();
+        var html = svc.BuildHtml(new[] { DefaultEntry }, MakeBranding(msp: true));
+        Assert.Contains("data:image/png;base64,bXNw", html);
+    }
 }