quote remote job paths in posix ssh command builders

[Samin061]

The POSIX builders in remote_job.py interpolate RemoteJobPaths values into single-quoted shell words without escaping, so a single quote anywhere in remote_base_dir closes the quote and the rest of the value is parsed as commands that run on the remote host as the SSH connection user. remote_base_dir is a template field, but _validate_base_dir runs in __init__ against the un-rendered Jinja literal and never looks for shell metacharacters, so a value like {{ dag_run.conf['dir'] }} reaches the shell unchecked and someone who can only trigger a Dag run with config gets a shell on the target host. Quoting with shlex.quote at each site matches what the Windows builders already do via ps_escape.

All six POSIX builders are affected. The _validate_job_dir prefix check on cleanup does not help, since a base dir that starts with /tmp/airflow-ssh-jobs/ and then contains a quote still passes it:

base_dir = "/tmp/airflow-ssh-jobs/x'; touch /tmp/pwned; :'"
build_posix_cleanup_command(paths.job_dir)
# rm -rf '/tmp/airflow-ssh-jobs/x'; touch /tmp/pwned; :'/job_123'

Added a parametrized regression that runs each builder's output through sh and asserts the injected marker is never created; it fails on all six before this change.

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