fix(gallery): clean up partially downloaded backend on installation failure (#8679)

When a backend download fails (e.g., on Mac OS with port conflicts causing
connection issues), the backend directory is left with partial files.
This causes subsequent installation attempts to fail with 'run file not
found' because the sanity check runs on an empty/partial directory.

This fix cleans up the backend directory when the initial download fails
before attempting fallback URIs or mirrors. This ensures a clean state
for retry attempts.

Fixes: #8016

Signed-off-by: localai-bot <localai-bot@users.noreply.github.com>
Co-authored-by: localai-bot <localai-bot@users.noreply.github.com>