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>

tests/unit/vector/test_errors.py

@@ -41,4 +41,4 @@ def test_format_nested_exception_group_flattens_all_leaves():
     assert "ValueError" in formatted
     assert "ConnectError" in formatted
     assert "RuntimeError" in formatted
-    assert "3 sub-exception(s)" in formatted
+    assert "3 sub-exceptions" in formatted