Enable Duo Chat custom rules by default

What does this MR do and why?

This MR enables the new feature by default so it can be released in 18.2 [Agentic Chat] Create custom rules - first iter... (#550743 - closed)

The flag has been set to true in all environments (e.g. gprd https://gitlab.com/gitlab-com/gl-infra/feature-flag-log/-/issues/45735).

References

Issue: [Agentic Chat] Create custom rules - first iter... (#550743 - closed)

How to set up and validate locally

You can follow the same setup as here but without enabling the FF in Rails Console.

!195829 (merged)

The graphql query should return "include_user_rule_context", in the available duo chat features.

MR acceptance checklist

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