Add review_merge_request_dap feature setting to decouple DAP code review model config

What does this MR do and why?

Introduces a dedicated review_merge_request_dap feature setting (ID 21) for the DAP code review flow so that Classic DCR (review_merge_request) and DAP code review have independent model selection entries in the admin UI.

Problem

Both flows currently resolve the same FeatureSetting record because WorkflowContextGenerationService#ai_feature returns :review_merge_request for code_review/v1. When the review_merge_request default was upgraded to Sonnet 4.6, Classic DCR timed out on large MRs (single blocking LLM call) while DAP handled it fine (agentic pipeline with compaction) — forcing both flows to be reverted together.

Solution

Following the same pattern used for the duo_agent_platform → duo_agent_platform_agentic_chat split (!213517 (merged)).

UI entry	Feature setting	ID	Flow
Code Review	review_merge_request	15	Classic DCR
Code Review (Agentic)	review_merge_request_dap	21	DAP code review
Agents & flows	duo_agent_platform	16	All other DAP flows

How to set up and validate locally

1. Run the migrations:

   bin/rails db:migrate

2. Verify the three tables have a row for review_merge_request_dap (feature = 21) copied from review_merge_request (feature = 15):

   # Rails console
   Ai::FeatureSetting.find_by(feature: :review_merge_request_dap)
   Ai::ModelSelection::NamespaceFeatureSetting.where(feature: :review_merge_request_dap).count
   Ai::InstanceFeatureSetting.where(feature: :review_merge_request_dap).count

3. Navigate to Admin area → AI-powered features and confirm a "Code Review (Agentic)" entry appears alongside "Code Review" with independent model selection.

4. Enable the feature flag and verify WorkflowContextGenerationService#ai_feature returns the new symbol for a DAP code review workflow:

   # Rails console
   Feature.enable(:review_merge_request_dap_model_config)
   
   user = User.find_by(username: 'your-username')
   service = Ai::DuoWorkflows::WorkflowContextGenerationService.new(
     current_user: user,
     workflow_definition: Ai::Catalog::FoundationalFlow['code_review/v1'].foundational_flow_reference
   )
   service.send(:ai_feature) # => :review_merge_request_dap

5. Disable the flag and confirm the method falls back to :review_merge_request:

   Feature.disable(:review_merge_request_dap_model_config)
   service.send(:ai_feature) # => :review_merge_request

Local Testing

Classic DCR	Code Review DAP flow	LS trace with FF disabled (used Sonnet 4)	LS trace with FF enabled (used Sonnet 4.6)
		Trace	Trace

References

• Classic Duo Code Review: support per-feature mo... (#598179)
• AIGW companion MR (deploy first): feat: Add review_merge_request_dap feature sett... (gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!5555 - merged)