When running `test-filters --parameters-file=...`, the parameters file's
--source entries were dropped and its --target was injected as the (only)
positional path, so filter testing ran against the backend URL instead of the
real source paths. The parameters-file handling only appended --source for the
`backup` command and always treated the first positional argument as a target.
test-filters/test-filter take source paths as positional arguments and have no
target, so the handling now appends --source for those commands (as it does for
backup) and does not inject --target (reported at verbose level, consistent with
how a --source on a non-backup command is already handled).
Adds an Issue4812 regression test driving Program.RunCommandLine.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
