feat(provider): add SNMP provider with GET/WALK support and pull alerts

[haoyousun60-create]

Summary

Adds a new SNMP Provider to Keep for monitoring network devices via SNMP (v1, v2c, v3).

Files

• keep/providers/snmp_provider/snmp_provider.py - Main provider (GET, WALK, pull_alerts, _format_alert)
• keep/providers/snmp_provider/init.py - Module exports
• keep/providers/snmp_provider/alerts_mock.py - Mock data for tests
• keep/providers/snmp_provider/README.md - Documentation
• pyproject.toml - Added pysnmp-lextudio dependency

Features

• GET/WALK: Query single OIDs or walk subtrees
• Pull Alerts: Polls IF-MIB and HOST-RESOURCES-MIB for non-normal statuses
• Trap Handling: _format_alert() processes incoming SNMP traps
• Auth: SNMP v1/v2c (community) and v3 (USM: MD5/SHA, DES/AES)

Closes

Fixes #3971

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