feat: Add SNMP provider for receiving SNMP traps as alerts

[ska2704]

Closes #2112

Description

Adds SNMP provider that receives SNMP v1/v2c traps as Keep alerts.

• keep/providers/snmp_provider/snmp_provider.py, provider with _format_alert(), severity mapping for all 6 generic trap types (coldStart, warmStart, linkDown, linkUp, authenticationFailure, egpNeighborLoss), SHA-256 fingerprinting for deduplication
• keep/providers/snmp_provider/alerts_mock.py - 6 realistic mock trap scenarios
• keep/providers/snmp_provider/__init__.py

Checks

• My pull request adheres to the code style of this project
• My code requires changes to the documentation
• I have updated the documentation as required
• All the tests have passed

Additional Information

SNMP is push-only network devices send traps to Keep's webhook endpoint, no credentials needed. The provider follows the same pattern as NetdataProvider (no auth config, pure _format_alert ingest).

/claim #2112

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Note

Low Risk
Adds a new provider and mock data without changing existing provider logic or shared infrastructure; primary risk is payload-format mismatches causing incorrect alert fields/severity.

Overview
Introduces a new SnmpProvider that accepts push-based SNMP trap webhook payloads and maps them into Keep AlertDtos, including generic-trap name/severity mapping, label extraction (community/agent/OID/varbinds), timestamp handling, and SHA-256 fingerprinting for deduplication.

Adds alerts_mock.py with several realistic SNMP trap examples to support BaseProvider.simulate_alert() for local testing/simulation (with an empty __init__.py scaffold for the new provider package).

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