fix: decode encoded Windows drive separators

[Bortlesboat]

Summary

• recognize a percent-encoded Windows drive separator before url2pathname performs drive detection
• avoid abspath prepending the current drive to an already-decoded drive path
• cover parity between file:///C:/... and file:///C%3A/...

Closes #2208

Verification

• PYTHONPATH=packages/markitdown/src python -m pytest packages/markitdown/tests/test_module_misc.py::test_file_uri_with_percent_encoded_drive_colon -q (1 passed on Windows)
• python -m ruff check packages/markitdown/src/markitdown/_uri_utils.py packages/markitdown/tests/test_module_misc.py
• python -m ruff format --check packages/markitdown/src/markitdown/_uri_utils.py packages/markitdown/tests/test_module_misc.py
• git diff --check

The existing test_file_uris case hard-codes POSIX paths and therefore does not run successfully on Windows; the new regression is isolated with a Windows marker.