feat: add Nagios XI Provider - fixes #3960

[derekliutao-cmd]

Summary

Implements a polling-based Nagios XI Provider following the CentreonProvider pattern, as requested in #3960.

• Polls host status (/api/v1/objects/hoststatus) and service status (/api/v1/objects/servicestatus) via Nagios XI REST API
• Uses apikey authentication
• Maps Nagios state codes (0=OK, 1=WARNING, 2=CRITICAL, 3=UNKNOWN) to Keep alert status/severity
• Follows CentreonProvider polling pattern: _get_alerts() calls __get_host_status() and __get_service_status(), errors in one don't block the other
• Handles single-object responses (API returns dict instead of list when only one host/service exists)
• Includes comprehensive unit tests with mocked HTTP responses
• Includes provider README with setup instructions and troubleshooting

Files Added

• keep/providers/nagiosxi_provider/__init__.py
• keep/providers/nagiosxi_provider/nagiosxi_provider.py
• keep/providers/nagiosxi_provider/README.md
• tests/test_nagiosxi_provider.py

Test Plan

• Host status mapping (UP->RESOLVED/LOW, DOWN->FIRING/CRITICAL, UNREACHABLE->FIRING/WARNING)
• Service status mapping (OK->RESOLVED/LOW, WARNING->FIRING/WARNING, CRITICAL->FIRING/CRITICAL, UNKNOWN->FIRING/INFO)
• Combined alerts (3 hosts + 4 services = 7 total)
• Error resilience (host failure doesn't block service alerts and vice versa)
• Single-object response handling
• Empty response handling
• Scope validation (authenticated)
• Config validation

Fixes #3960

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[shahargl]

N/A