Revert "Fix Windows path separator issue in paired notebooks (#1454)"

This reverts commit 3b47cde but keeps the added test

Author: mwouts

src/jupytext/paired_paths.py

@@ -61,15 +61,7 @@ def join(left, right, sep):
 
 def separator(path):
     """Return the local path separator (always / in the contents manager)"""
-    # On Windows, determine the appropriate separator
-    if os.path.sep == "\\":
-        # If path contains \, definitely use Windows separator
-        if "\\" in path:
-            return "\\"
-        # If path contains / but not \, it's probably a format string or contents manager path
-        if "/" in path:
-            return "/"
-        # Path has no separators (e.g., bare filename) - use OS separator
+    if os.path.sep == "\\" and "\\" in path:
         return "\\"
     return "/"
 

tests/unit/test_paired_paths.py

@@ -101,7 +101,7 @@ def test_paired_paths_windows_no_subfolder():
 
 def test_paired_paths_windows_relative():
     """Test Windows pairing with relative paths and backslash as path separator, issue #1028"""
-    nb_file = "notebooks\\example.ipynb"
+    nb_file = "C:\\notebooks\\example.ipynb"
     formats = "notebooks///ipynb,scripts///py:percent"
     with mock.patch("os.path.sep", "\\"):
         # Should not raise InconsistentPath
@@ -110,8 +110,8 @@ def test_paired_paths_windows_relative():
         path_list = [p[0] for p in paths]
         assert nb_file in path_list, f"Expected {nb_file} to be in paired paths {path_list}"
         # Verify both paths use backslashes on Windows
-        assert paths[0][0] == "notebooks\\example.ipynb"
-        assert paths[1][0] == "scripts\\example.py"
+        assert paths[0][0] == "C:\\notebooks\\example.ipynb"
+        assert paths[1][0] == "C:\\scripts\\example.py"
 
 
 @pytest.mark.parametrize("os_path_sep", ["\\", "/"])