Add support for group-level Duo Code Review custom instructions

What does this MR do and why?

This merge request adds support for GitLab Duo Code Review to inherit custom review instructions from groups, not just individual projects.

Previously, code review instructions could only be defined at the project level. Now, groups can set up template projects with review instructions that automatically apply to all projects within that group and its subgroups. This creates a hierarchy where instructions from higher-level groups are combined with project-specific instructions.

This code is in use yet and a follow up MR will introduce an API endpoint do Duo Workflow Service can fetch these instructions for DAP mode.

The same will apply for the classic mode, where ReviewMergeRequest will have to be updated.

References

Related to #595535 (closed)

How to set up and validate locally

1. Add custom instructions to a project
2. Create a merge request under that project
3. In a Rails console, fetch the project and merge_request and run Gitlab::Duo::CodeReview::CustomInstructionsResolver.new(project).execute(merge_request)
   • Make sure instructions apply to the files (extensions) in the merge request you just created
   • This should return the projects instructions only
4. Create a second project under the same group and add more instructions
   • This second project will serve as a template for custom instructions for all projects under that group
5. Configure the group template: project.group.create_namespace_template_setting(duo_template_project: duo_template_project)
   • Where duo_template_project is the project you created on step 4)
6. Enable duo_code_review_group_level_instructions feature flag: ::Feature.enable(:duo_code_review_group_level_instructions, project)
7. Fetch the instructions again: Gitlab::Duo::CodeReview::CustomInstructionsResolver.new(project).execute(merge_request)

pry(main)> Gitlab::Duo::CodeReview::CustomInstructionsResolver.new(project).execute(merge_request)
=> [#<Gitlab::Duo::CodeReview::CustomInstruction:0x0000000146a023b8
  @exclude_patterns=[],
  @include_patterns=["**/*.go"],
  @instructions="1. Make any suggestions using Shakespeare conversation style",
  @location="http://gdk.test:3000/gitlab-duo/duo-subgroup/duo-template/-/blob/main/.gitlab/duo/mr-review-instructions.yaml",
  @name="Go files">]

Database

Query plan: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/50595/commands/150093

Unfortunately there are no groups with a Duo template project just yet.

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