Fix regression: only apply cache isolation in parallel (xdist) mode

Co-authored-by: shaypal5 <917954+shaypal5@users.noreply.github.com>

Author: Copilot

tests/conftest.py

@@ -104,10 +104,14 @@ def isolated_cache_directory(tmp_path, monkeypatch, request, worker_id):
     This is especially important for pickle and maxage tests when running in parallel. Each pytest-xdist worker gets its
     own cache directory to avoid conflicts.
 
+    Only applies when running in parallel mode (pytest-xdist), to avoid breaking tests that use module-level path
+    constants computed from the default cache directory at import time.
+
     """
-    if "pickle" in request.node.keywords or "maxage" in request.node.keywords:
-        # Create a unique cache directory for this test
-        cache_dir = tmp_path / "cachier_cache" if worker_id == "master" else tmp_path / f"cachier_cache_{worker_id}"
+    if worker_id != "master" and (
+        "pickle" in request.node.keywords or "maxage" in request.node.keywords
+    ):
+        cache_dir = tmp_path / f"cachier_cache_{worker_id}"
 
         cache_dir.mkdir(exist_ok=True, parents=True)