🛡️ Sentinel: [security improvement] Sanitize IPv6 scope IDs

[ManupaKDU]

🚨 Severity: MEDIUM
💡 Vulnerability: The Python ipaddress module allows arbitrary characters (including newlines, spaces, and control characters) within the scope_id of an IPv6Address.
🎯 Impact: If str(ip_obj) is called and the resulting string is passed to subprocess.call or logged, it introduces risks of Log Injection and unpredictable Argument Injection.
🔧 Fix: Added strict validation in is_reachable using re.fullmatch to ensure the scope_id only contains alphanumeric characters and hyphens.
✅ Verification: Added test_is_reachable_invalid_scope_id unit test to confirm the validation blocks malformed inputs and prevents subprocess.call execution.

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PR created automatically by Jules for task 12319616566019819454 started by @ManupaKDU

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