test(webdav): pin encode-once contract + nit cleanups (#891 r3)

Round-3 review on PR #891 (no blockers):
- Add test_encode_dav_path_encodes_exactly_once pinning the documented
  decoded-input precondition ("already%20encoded.pdf" -> "already%2520...").
- format_exception_group: proper singular/plural ("1 sub-exception" vs
  "N sub-exceptions") instead of "(s)".
- oauth_sync: use `if doc_task is not None:` to match processor_task's guard.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/vector/oauth_sync.py

@@ -332,7 +332,7 @@ async def multi_user_processor_task(
             break
 
         except NotProvisionedError:
-            if doc_task:
+            if doc_task is not None:
                 logger.warning(
                     "[BasicAuth] User %s not provisioned, skipping %s_%s",
                     doc_task.user_id,
@@ -342,7 +342,7 @@ async def multi_user_processor_task(
             continue
 
         except Exception as e:
-            if doc_task:
+            if doc_task is not None:
                 logger.error(
                     "[BasicAuth] Processor %s error processing %s_%s: %s",
                     worker_id,