Add merge_request_iid support to PipelineCreate mutation

What does this MR do and why?

Adds merge_request_iid support to PipelineCreate mutation to create pipelines associated with merge requests through the GraphQL

When merge_request_iid is provided:

• The mutation finds the merge request by IID
• Passes it to Ci::CreatePipelineService
• Creates a pipeline linked to the merge request

This MR adds the backend changes required to make the frontend changes in !225516 (merged) functional, which adds the ability to create MR pipelines with inputs.

References

Relates to #547861

How to set up and validate locally

1. Verify the entire flow using the frontend changes at !225516 (merged) OR the following
2. Enable the feature flag: Feature.enable(:enable_inputs_for_mr_pipelines)
3. Create a test merge request (or use an existing one):
   • Note the project path and MR IID
4. Ensure the project has a valid .gitlab-ci.yml
5. Create a pipeline for an MR:

mutation { 
  pipelineCreate(input: { 
    projectPath: "your-group/your-project" 
    ref: "feature-branch" 
    mergeRequestIid: "123" 
  }) { 
    pipeline { 
      id 
      iid 
      source 
      mergeRequest { 
        iid 
      } 
    } 
    errors 
  } 
}

1. Verify the response:
   • pipeline.source should be "merge_request_event"
   • pipeline.mergeRequest.iid should match the provided IID

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #547861