AI Gateway - AI Gateway log IDs and request IDs now use UUIDv7

New AI Gateway log IDs and request IDs are lowercase UUIDv7, such as 019a7b3e-5f2c-7c41-9d3e-2b8f6a1c4e07. Previously, log IDs were 26-character ULIDs and request IDs were random UUIDs.

For a request that AI Gateway answers in a single step, the cf-aig-request-id, cf-aig-log-id, and cf-aig-event-id response headers now carry the same value. For provider and Universal requests, if you set your own cf-aig-event-id, AI Gateway returns it instead.

Existing log IDs do not change, and the logs API accepts both formats. Treat log IDs as opaque strings, and update any code that checks for the ULID format.