Enable duo_developer_model_config feature flag by default

What does this MR do and why?

Enables the duo_developer_model_config feature flag by default by changing the flag type from gitlab_com_derisk to beta and setting default_enabled: true.

This feature routes the DAP developer flow to the duo_developer feature setting instead of the shared duo_agent_platform feature setting, so its model config can be changed independently. It was introduced in GitLab 19.4 and has been successfully derisk-tested on GitLab.com.

Related issue: gitlab-org#23227 (closed) Rollout issue: #623124 (closed)

MR acceptance checklist

• Feature flag type changed from gitlab_com_derisk to beta
• default_enabled set to true
• Documentation updated with history entry (no documentation file references this feature flag)

References

• Rollout issue: #623124 (closed)
• Introduced by: !251961 (merged)
• Pattern followed from: !230928 (closed)