Implement TH-07 security test validating that PDF password ingress
channels properly prevent password disclosure via process arg list.
Test cases:
- --password VALUE rejected with exit 64 without opt-in
- --password VALUE with PDFTRACT_INSECURE_CLI_PASSWORD=1 proceeds with warning
- --password-stdin works correctly
- PDFTRACT_PASSWORD env var works correctly
- Password leaks in /proc/<pid>/cmdline under opt-in (proving the vulnerability)
- Password does NOT leak with --password-stdin or env var
Closes: pdftract-43jxa
