This PR fixes an Ubuntu CI failure in Opera driver install flow:

NotADirectoryError: .../.wdm/drivers/operadriver/.../operadriver

Root cause:

• stale cache can contain a file where a new archive expects a directory
  (file/dir name collision at top-level archive entries).
• zip.extractall() then fails with NotADirectoryError.

Fix:

• In FileManager.__extract_zip, pre-scan archive entries and remove stale
  conflicting files in target directory before extraction.

Added regression test:

• tests/test_file_manager.py::test_extract_zip_removes_file_dir_conflict
  reproduces the file-vs-directory collision and verifies extraction succeeds.

Why this is safe:

• cleanup is narrowly scoped to top-level paths required as directories by the archive.
• does not alter normal extraction flow when no conflict exists.