diag(attachments): post-upload "did the file actually arrive" check

The server reported successful uploads for the two PDFs into case 6a03166d…
("מונאס אריאל") but the user couldn't find the files on the share. Both
LocalFilesystemClient::uploadFile and WebDavClient::uploadFile return
true only on a real write, so something downstream is silently dropping
the payload — most likely a WebDAV proxy or a basePath mismatch
between what NetworkStorageIntegration thinks the storage is vs. what
the user is browsing.

Add a verification step right after the upload:

  1. POST /Attachment   →  attachmentId
  2. POST /NetworkStorage/action/upload  →  201 from server
  3. GET  /NetworkStorage/folderContents?path=<folder>
     • If our filename is in the listing → INF "verify present"
     • Else → WRN "expected X but NOT in listing. Currently in folder: […]"

The diagnostic line surfaces both halves of the puzzle in a single
log entry, so the next save attempt will tell us in plain text whether
the storage layer is the liar or the path is being normalised
somewhere unexpected.

Tests: 39 passing.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/OutlookAddin.Core/Services/AttachmentSaveService.cs

@@ -59,6 +59,12 @@ namespace MarcusLaw.OutlookAddin.Core.Services
                     await _client.UploadAttachmentToNetworkStorageAsync(
                         uploaded.Id, trimmedFolder, cancellationToken).ConfigureAwait(false);
 
+                    // Step 3: verify the file actually landed on the share —
+                    // the server can return 200 yet leave nothing on disk
+                    // when a downstream WebDAV proxy silently fails. Read
+                    // the folder back and look for our filename.
+                    await VerifyArrivedAsync(att.Name, trimmedFolder, cancellationToken).ConfigureAwait(false);
+
                     _logger.Information(
                         "Saved attachment {Name} → {Path} (attachmentId={AttachmentId})",
                         att.Name, trimmedFolder, uploaded.Id);
@@ -99,5 +105,54 @@ namespace MarcusLaw.OutlookAddin.Core.Services
 
             return summary;
         }
+
+        /// <summary>
+        /// Post-upload sanity check: list the folder we just wrote into
+        /// and confirm a file with the same (possibly sanitized) name
+        /// shows up. Diagnostic only — does not change the outcome. If
+        /// the file isn't there, log a WARNING with everything that IS
+        /// in the folder so we can spot the actual server-side path.
+        /// </summary>
+        private async Task VerifyArrivedAsync(string expectedName, string folderPath, CancellationToken ct)
+        {
+            try
+            {
+                var items = await _client.ListNetworkStorageFolderAsync(folderPath, ct).ConfigureAwait(false);
+                var trimmedExpected = expectedName.Trim().TrimStart('-', ' ').TrimEnd('-', ' ');
+                bool found = false;
+                foreach (var it in items)
+                {
+                    if (it.IsFolder) continue;
+                    if (string.Equals(it.Name, expectedName, StringComparison.Ordinal) ||
+                        string.Equals(it.Name, trimmedExpected, StringComparison.Ordinal))
+                    {
+                        found = true;
+                        break;
+                    }
+                }
+                if (!found)
+                {
+                    var names = new List<string>();
+                    foreach (var it in items)
+                    {
+                        if (it.IsFolder) continue;
+                        names.Add(it.Name);
+                    }
+                    _logger.Warning(
+                        "Post-upload verify: expected '{Expected}' under '{Path}' but it's NOT in the listing. " +
+                        "Server reported the upload as successful but the file is not on the share. " +
+                        "Files currently in that folder: [{Files}]",
+                        expectedName, folderPath, string.Join(", ", names));
+                }
+                else
+                {
+                    _logger.Information("Post-upload verify: '{Expected}' present under '{Path}'", expectedName, folderPath);
+                }
+            }
+            catch (Exception ex)
+            {
+                _logger.Warning(ex, "Post-upload verify failed for '{Expected}' under '{Path}'", expectedName, folderPath);
+            }
+        }
     }
 }