Introduce dap_web_search feature flag

What does this MR do and why?

Introduces the dap_web_search feature flag to enable web search capabilities for GitLab Duo.

Related Issues and MRs

• Feature Flag Issue: [FF] `dap_web_search` -- Controls Duo Chat web ... (#587158)
• Related issue: Web Search in Duo Chat (gitlab-org#20480)
• DWS Implementation MR: feat: enable web search for chat workflow (gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!4299 - merged)