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/client/test_webdav.py

@@ -540,6 +540,15 @@ def test_webdav_path_encoding(path, expected):
     assert client._webdav_path(path) == expected
 
 
+@pytest.mark.unit
+def test_encode_dav_path_encodes_exactly_once():
+    """Pins the decoded-input precondition: a literal '%' becomes '%25', so an
+    already-encoded path passed in error would double-encode (caught here)."""
+    from nextcloud_mcp_server.client.webdav import _encode_dav_path
+
+    assert _encode_dav_path("already%20encoded.pdf") == "already%2520encoded.pdf"
+
+
 @pytest.mark.unit
 async def test_read_file_encodes_special_chars(mocker):
     """read_file must percent-encode '#', commas, and spaces in the path (card 309).

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