POC: Solve duplicate MR pipelines

What does this MR do and why?

POC: Add prevent_duplicate_mr_pipelines project setting

When pushing to a branch with an open merge request, GitLab creates both a branch pipeline and a merge request pipeline. This causes confusion, wastes CI/CD resources, and creates compliance gaps.

Add a new opt-in project setting prevent_duplicate_mr_pipelines.

When enabled:

• The branch pipeline is suppressed at BaseHooksService#create_pipeline? before any CI config parsing or Sidekiq job is enqueued.
• The implicit only: [branches, tags] default is stripped from jobs without explicit rules/only/except, so they are included in merge request pipelines.
• Only push-triggered pipelines are affected. Manual, API, scheduled, and triggered pipelines are unaffected.

References

Create at most 1 pipeline when in the context o... (gitlab-org#6044)

How to set up and validate locally

1. Create an invalid CI Config to a project

2. Create an MR with some changes

3. See the duplicate pipelines

4. Make another change on the MR branch

5. See the duplicate pipelines again

6. Enable the setting

7. Make another change on the MR branch

8. See no duplicate pipelines

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