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/_errors.py

@@ -21,7 +21,8 @@ def format_exception_group(exc: BaseException) -> str:
     if not isinstance(exc, BaseExceptionGroup):
         return repr(exc)
     leaves = _flatten(exc)
-    return f"{len(leaves)} sub-exception(s): " + "; ".join(repr(e) for e in leaves)
+    noun = "sub-exception" if len(leaves) == 1 else "sub-exceptions"
+    return f"{len(leaves)} {noun}: " + "; ".join(repr(e) for e in leaves)
 
 
 def _flatten(exc: BaseException) -> list[BaseException]: