[Bug] Azure integration missing event.category across events

## Summary

Azure integration has missing `event.category` for below events. This is a required ECS field for Elastic Security and its absence breaks EQL-based threat detection rules.

## Current behavior
`event.category` is not populated on a significant subset of events ingested by the azure integration. This causes EQL detection rules that filter on `event.category` to silently miss matching events, reducing detection coverage.

### Environment
Azure Logs v1.36.1

- azure.auditlogs
- azure.eventhub
- azure.activitylogs

### Additional Notes
More context: https://github.com/elastic/integrations/issues/18343

Start by inspecting the azure.auditlogs, azure.eventhub, and azure.activitylogs event mappings for the affected events. Compare their output with the ECS requirement for event.category and verify that EQL-based detection rules no longer miss events once the field is populated across the listed integrations.

Written by the indexing model from the issue text.