Fix full FTP URLs in direct file downloads

[rioyu123]

Summary

Direct FTP reads and downloads did not handle full URLs consistently. cat_file("ftp://host:port/file") and get_file() passed the entire URL to RETR, so the server could not find the file. Reads with an end or a negative start already worked because they went through the buffered-file path.

Strip the protocol before either direct download path sends its command, using the same normalization as the other FTP methods.

This also makes direct relative paths start from /, matching the other FTP methods.

Tests

• Added cases for full and partial reads, plus downloads to a local path and a file-like object, using the existing local FTP server fixture.
• Before the fix: four new cases failed; the two buffered-read controls passed.
• After the fix: all 39 FTP tests passed; pytest fsspec/tests -q --ignore=fsspec/tests/test_downstream.py: 884 passed, 11 skipped, 2 xfailed.
• Changed-file Ruff checks and wheel/sdist builds passed.