Add AI gateway timeout setting to admin page

What does this MR do and why?

Adds the new AI gateway timeout setting (introduced in Add `ai_gateway_timeout_seconds` column in `ai_... (!211294)) in the admin GitLab Duo setting page.

References

Update Admin Duo settings form to add new AIGW ... (#579183)

Bug issue: Make 60s timeout applied by AIGW when making re... (#567878)

Screenshots or screen recordings

demo

How to set up and validate locally

Prerequisites:

• As this MR requires on database changes in Add `ai_gateway_timeout_seconds` column in `ai_... (!211294), a migration needs to be run prior: bin/rails db:migrate

Steps:

• In admin/gitlab_duo/configuration, observe new setting Ai Gateway timeout request.
• Updating to value within range of 60-600 works.
• Validation when updating value with value less than 60 or greater than 600.

MR acceptance checklist

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