fix: Enable Zabbix ID deduplication

[mvanhorn]

Closes #4767

📑 Description

Declare id as ZabbixProvider's default fingerprint field, following the existing Graylog, Wazuh, Checkmk, and Dynatrace provider pattern. Keep the change at the provider contract boundary: ProvidersFactory.get_all_providers() already copies this declaration into default_fingerprint_fields, and get_default_deduplication_rules() already turns non-empty values into enabled provider defaults. Add factory-level regression coverage that verifies Zabbix metadata exposes id and that the generated Zabbix default rule uses it, while isolating the factory caches so the assertions cannot pass because of earlier test state.

Zabbix alerts carry their problem occurrence identifier in the normalized alert id, but ZabbixProvider does not declare a default fingerprint field. Provider discovery therefore publishes no default fingerprint fields for Zabbix, so the deduplication service omits the provider's default rule and a Zabbix-only installation can show an empty deduplication page. Graylog already declares FINGERPRINT_FIELDS = ["id"] in the current tree, leaving Zabbix as the concrete unresolved case described by the latest human report. The existing custom-rule API workaround confirms that id is accepted by the deduplication path once supplied.

✅ Checks

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

ℹ Additional Information

Nothing beyond what is described above.

[shahargl]

lgtm

[github-actions]

🌟 Well done @mvanhorn! Two PRs merged already! 🎉🥳

With your second PR, you're on a roll, and your contributions are already making a difference. 🌟
Looking forward to seeing even more contributions from you. See you in Slack https://slack.keephq.dev 🚀