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/HtmlExportServiceTests.cs

@@ -15,6 +15,13 @@ public class HtmlExportServiceTests
         string url = "https://contoso.sharepoint.com/sites/A") =>
         new("Web", "Site A", url, true, users, userLogins, "Read", "Direct Permissions", "User");
 
+    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);
+    }
+
     [Fact]
     public void BuildHtml_WithKnownEntries_ContainsUserNames()
     {
@@ -50,4 +57,34 @@ public class HtmlExportServiceTests
         // The HTML should surface the external marker so admins can identify guests
         Assert.Contains("EXT", html, StringComparison.OrdinalIgnoreCase);
     }
+
+    // ── Branding tests ────────────────────────────────────────────────────────
+
+    [Fact]
+    public void BuildHtml_WithMspBranding_ContainsMspLogoImg()
+    {
+        var entry = MakeEntry("Test", "test@contoso.com");
+        var svc = new HtmlExportService();
+        var html = svc.BuildHtml(new[] { entry }, MakeBranding(msp: true, client: false));
+        Assert.Contains("data:image/png;base64,bXNw", html);
+    }
+
+    [Fact]
+    public void BuildHtml_WithNullBranding_ContainsNoLogoImg()
+    {
+        var entry = MakeEntry("Test", "test@contoso.com");
+        var svc = new HtmlExportService();
+        var html = svc.BuildHtml(new[] { entry });
+        Assert.DoesNotContain("data:image/png;base64,", html);
+    }
+
+    [Fact]
+    public void BuildHtml_WithBothLogos_ContainsTwoImgs()
+    {
+        var entry = MakeEntry("Test", "test@contoso.com");
+        var svc = new HtmlExportService();
+        var html = svc.BuildHtml(new[] { entry }, MakeBranding(msp: true, client: true));
+        Assert.Contains("data:image/png;base64,bXNw", html);
+        Assert.Contains("data:image/jpeg;base64,Y2xpZW50", html);
+    }
 }