feat: Add SNMP provider with v1/v2c/v3 polling and trap receiving

[vasquezmyguy-collab]

SNMP Provider for Keep

Implements #2112 — SNMP provider with full v1/v2c/v3 support.

Features

• Polling mode: SNMP GET for configurable OIDs using pysnmp.hlapi.asyncio.getCmd, with asyncio.run() wrapper for sync compatibility with Keep's _query() interface
• Trap receiver: Background daemon thread with its own asyncio event loop, using pysnmp.entity low-level API + ntfrcv.NotificationReceiver with proper 5-arg callback
• SNMP v1/v2c/v3: Full auth support via CommunityData and UsmUserData with configurable auth/priv protocols (MD5, SHA, SHA224/256/384/512, DES, 3DES, AES128/192/256)
• Well-known trap OID mappings: Automatic severity classification for linkDown, coldStart, warmStart, authenticationFailure, egpNeighborLoss, enterpriseSpecific
• Standard Keep provider interface: validate_config(), validate_scopes(), dispose(), status(), PROVIDER_SCOPES, PROVIDER_CATEGORY, PROVIDER_TAGS
• Mock alert data: 6 trap scenarios for testing

File structure

keep/providers/snmp_provider/
├── __init__.py          # Empty
├── snmp_provider.py    # Main provider class (~800 lines)
└── alerts_mock.py      # Mock alert data

/claim #2112

Closes #2112

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