Fix AI Feature Attribution for Customer Billing

Problem to solve

AI Gateway reporting shows high percentage of null values for feature_enabled_by_namespace_ids field. This prevents:

• Accurate customer billing for AI features
• Sales teams from showing customers their usage data
• Proper revenue attribution

Why now?

• Events in Duo Agent Platform (like receive_start_duo_workflow) are missing the feature_enabled_by_namespace_ids field
• Field teams cannot report AI adoption/utilization to customers
• Revenue attribution is broken

Proposed solution

Add feature_enabled_by_namespace_ids to the InternalEventAdditionalProperties for Duo Agent Platform events in server.py

Definition of done

• feature_enabled_by_namespace_ids field is added to all Duo Agent Platform events
• Field correctly captures which namespace enabled/pays for the feature (not just where it's used)
• Percentage of null values for this field drops significantly (target: <10%)
• AI Gateway can properly attribute usage to paying customers