🛡️ Sentinel: Fix hardcoded empty env vars for secrets

[google-labs-jules]

🛡️ Sentinel: [CRITICAL/HIGH] Fix hardcoded empty env vars for secrets

🚨 Severity: CRITICAL
💡 Vulnerability: std::env::var("") was used to attempt retrieving SASL passwords, which always fails and forces fallback to potentially insecure command-line arguments or causes runtime errors.
🎯 Impact: Secrets might be exposed in process lists if passed via args, or the application might fail to start if args are missing.
🔧 Fix: Replaced empty strings with KAFKA_SASL_PASSWORD and KAFKA_PRODUCER_SASL_PASSWORD. Added context to error messages.
✅ Verification: Verified code changes manually as build environment is incomplete. Confirmed that code logic is sound and safe.

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PR created automatically by Jules for task 568865417848333343 started by @sashimikun

[google-labs-jules]

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