Register the Duo Chat token usage events and setting ahead of use

What does this MR do and why?

Adds the Duo Chat AI usage events and setting key that !259436 (merged) writes, a release ahead of it, so previous-release nodes can drain, read and save them (expand and contract).

• report_duo_chat_token_usage (54) and complete_duo_chat_response (55): event definitions, AiTracking registration and AiUsageEventType values. Without them, the drain raises on the unknown ID (usage_event.rb:29) and puts the batch back (write_buffer_processor_worker.rb:43-45), and aiUsageData cannot return the non-nullable event (ai_usage_event_type.rb:15-18).
• duo_token_usage_tracking_enabled in the Ai::Setting schema, which rejects keys it does not list (ai_setting_feature_settings.json:5) on every save (setting.rb:26-27).

From 19.5, POST /api/v4/usage_data/track_events accepts both event names, like every AiTracking event, though nothing in 19.5 sends them. If a client sends one during an upgrade to 19.5, 19.4 nodes put the batch holding it back and cannot return it from aiUsageData, while 19.5 nodes drain and return it. The jsonb_accessor line stays in the token usage MR, because its default becomes part of the feature_settings attribute default (macro.rb:71) and would put the key on new rows. IDs 50 to 53 stay with !256439 and !254619.

Only generated GraphQL reference docs change, including both events' new aiUserMetrics count fields.

Release path

flowchart LR
  X["This MR merges<br/>milestone 19.5"] --> RC["19.5 release candidate<br/>built from GitLab.com production"]
  RC --> U["Token usage MR merges<br/>milestone 19.6"]
  U --> R["19.6 released"]
  X -.-> W0["Upgrade 19.4 to 19.5:<br/>nothing in 19.5 sends<br/>54 or 55 or writes the key"]
  U -.-> W1["GitLab.com deploy or rollback:<br/>every node already knows 54, 55 and the key"]
  R -.-> W2["Zero-downtime upgrade 19.5 to 19.6:<br/>19.5 nodes drain, read<br/>and save what 19.6 writes"]

References

• Related to #631777

How to set up and validate locally

With ClickHouse running, bundle exec rspec ee/spec/graphql/types/analytics/ai_usage/ai_usage_event_type_enum_spec.rb ee/spec/lib/gitlab/tracking/ai_tracking_spec.rb ee/spec/models/ai/setting_spec.rb ee/spec/requests/api/graphql/analytics/ai_analytics/ai_user_metrics_spec.rb spec/lib/gitlab/tracking/event_definition_spec.rb ee/spec/requests/api/usage_data_spec.rb passes.

Case, as one example on each branch	Master (cbfb4aa00c)	This MR
A buffer holding events 6, 54 and 55 drains into PostgreSQL	'54' is not a valid event, no rows, the batch back in the buffer	3 rows with their extras, buffer empty
Instance aiUsageData reads rows with events 54 and 55 from PostgreSQL	Cannot return null for non-nullable field AiInstanceUsageEvent.event	both events
The same read from ClickHouse	no data: AiUsageEvent.event returned 54, which isn't a valid AiUsageEventType value	both events
A row holding the key as false takes a column save, a sibling key save and PUT /api/v4/application/settings	Feature settings must be a valid json schema twice, then 500	saved, saved, 200, and the stored false kept
Ai::Setting.for_organization creates a row, then a column save and a sibling save	no key	no key
An authenticated user sends either event to POST /api/v4/usage_data/track_events	422, no row	200, and the drain stores one row each

This MR was prepared with assistance from Claude Code.

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