🛡️ Sentinel: Fix password truncation for passwords with spaces

[google-labs-jules]

🛡️ Sentinel: [CRITICAL/HIGH] Fix password truncation

Severity: HIGH
Vulnerability: Unquoted variables in entrypoint.sh caused passwords containing spaces to be truncated (e.g., "my secret" became "my"). This could lead to weaker passwords than intended or correct passwords being rejected if they contained spaces.
Impact: Users setting passwords via the USERS environment variable would silently have their passwords truncated if they contained spaces.
Fix: Quoted the $username and $password variables in the adduser function calls and used read -r for safer string reading.
Verification: Added tests/verify_password_fix.sh which mocks the vpncmd environment and verifies that passwords with spaces are passed correctly to the underlying command.

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PR created automatically by Jules for task 15902443492047366560 started by @bluPhy

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