feat: add Nagios XI provider

[Treasure520520]

Fixes #3960

/claim #3960

Summary

Adds a Nagios XI provider using the REST API polling path requested in #3960.

This intentionally follows the BaseProvider/Centreon-style polling pattern instead of the webhook-only approach in #6492:

• adds NagiosProvider with Nagios XI API key authentication
• polls /nagiosxi/api/v1/objects/hoststatus and /nagiosxi/api/v1/objects/servicestatus
• supports host_url configured either as the server root or the /nagiosxi base path
• maps Nagios host and service state codes into Keep alert status/severity
• marks acknowledged firing problems as ACKNOWLEDGED
• adds provider docs, generated-style snippet, docs nav entry, and focused unit tests

Validation

• python -m json.tool docs/mint.json >/dev/null
• poetry run black --check keep/providers/nagios_provider/nagios_provider.py tests/test_nagios_provider.py
• poetry run ruff check keep/providers/nagios_provider/nagios_provider.py tests/test_nagios_provider.py
• poetry run python -m pytest tests/test_nagios_provider.py -q (5 passed; existing warnings only)
• git diff --check

[shahargl]

N/A