Introduce duo_chat_monolith_components FF

What does this MR do and why?

Introduces the duo_chat_monolith_components feature flag (gitlab_com_derisk type) to control the use of migrated Duo Chat view components from the GitLab monolith instead of the @gitlab/duo-ui package.

This MR adds:

• The feature flag YAML definition in config/feature_flags/gitlab_com_derisk/duo_chat_monolith_components.yml
• Frontend feature flag exposure via push_frontend_feature_flag in ee/lib/ee/gitlab/gon_helper.rb

The flag is disabled by default and scoped to current_user. The MR !228844 (merged) uses this feature flag. We are introducing the feature flag in a separate MR to reduce possibility of merge conflicts and expedite merging.

References

• Feature issue: #593464
• Rollout issue: #595622

Screenshots or screen recordings

Not applicable - no UI changes.

Before	After
N/A	N/A

How to set up and validate locally

1. Enable the feature flag:

   Feature.enable(:duo_chat_monolith_components)

2. Verify the flag is exposed to the frontend by checking gon.features.duoChatMonolithComponents in the browser console.

3. Disable the feature flag:

   Feature.disable(:duo_chat_monolith_components)

4. Confirm gon.features.duoChatMonolithComponents is no longer present.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #593464