feat: SNMP provider - trap listener + OID mapping

[zhaog100]

Closes #2112

SNMP Provider

• SNMPv2c & SNMPv3 support with auth (MD5/SHA) and privacy (DES/AES)
• Trap listener via pysnmp-lextudio NotificationReceiver on configurable host:port
• Optional polling - periodic OID queries against configured targets
• OID-to-alert mapping - JSON-configurable OID patterns → alert name + severity
• Clean lifecycle - daemon threads with dispose()
• Optional dependency - graceful fallback if pysnmp not installed

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