AI Gateway 2.1: support for Kong Identity Principals

AI Gateway 2.1 adds support for Kong Identity Principals for AI-specific rate limiting, access control, and analytics. We need to update the relevant AI Gateway documentation to explain where authenticated Principals can be used. The feature is additive and existing Consumer-based behavior remains unchanged.

Scope

NOTE: This is TBD with Greg and engineering

Document the customer-facing surfaces:

• AI Model Provider: configuring principal as the identifier for AI rate limiting, alongside consumer.
• Conditional access: using Principal metadata in existing condition fields, for example:

principal.metadata.tier == "gold"

• AI Analytics: using Principal as a dimension/filter for AI traffic and cost attribution.
• How Principal-based AI functionality relates to the existing Kong Identity authentication flow.
• Principal vs Consumer behavior where it is relevant to users.

Start by locating the relevant AI Gateway documentation and existing sections that describe Consumer-based behavior and Kong Identity authentication. Document Principal use for AI rate limiting, conditional access, analytics, and the Principal-versus-Consumer relationship; done means all listed customer-facing surfaces are covered without changing existing Consumer behavior.