Apply escapeshellarg() to the Postgres username before interpolating it
into the chown command used to fix SSL certificate ownership, matching
the handling already in place for StartMysql. This keeps the sink-side
escaping consistent across database actions, independent of upstream
input validation.
Also adjusts an assertion in DatabaseSslCredentialEscapingTest to match
the actual double-escaped output of executeInDocker, and adds Postgres
regression cases for subshell and semicolon payloads.
f0e955bf45