Initial reactive cache implementation for pipeline inputs

What does this MR do and why?

This MR implements reactive caching for the ciPipelineCreationInputs GraphQL query to improve performance when fetching pipeline inputs that may include external file references.

Problem: With the introduction of external files for pipeline inputs, the ciPipelineCreationInputs GraphQL query can become slow due to external file fetching (local, project, remote, etc.). This is similar to the performance issue we encountered with prefill variables.

Solution: Following the same pattern used in Ci::ListConfigVariablesService, this MR adds ReactiveCaching to Ci::PipelineCreation::FindPipelineInputsService. This caches the pipeline inputs calculation results, preventing repeated slow external file fetches.

References

• Caching ciPipelineCreationInputs (#581342) • Sahil Sharma • 18.9
• Feature flag rollout: [FF] `ci_pipeline_inputs_reactive_cache` -- Int... (#587022)
• Related pattern: app/services/ci/list_config_variables_service.rb

How to set up and validate locally

Setup

1. Create a test project with pipeline inputs:

   Create a .gitlab-ci.yml with inputs:

   spec:
     inputs:
       environment:
         type: string
         default: "production"
       deploy_enabled:
         type: boolean
         default: true
   
   stages:
     - deploy
   
   deploy:
     stage: deploy
     script:
       - echo "Deploying to $[[ inputs.environment ]]"
     rules:
       - if: $[[ inputs.deploy_enabled ]]

2. Enable the feature flag:

   # In Rails console
   Feature.enable(:ci_pipeline_inputs_reactive_cache)

Validation

1. Test cache miss behavior (default - silent):

   query {
     project(fullPath: "your/project") {
       ciPipelineCreationInputs(ref: "main") {
         name
         default
         required
       }
     }
   }

   • First request returns null (cache miss, worker enqueued)
   • Subsequent requests return the cached inputs

2. Test cache miss behavior (fail on miss):

   • Reset the existing cache by updating the inputs in .gitlab-ci.yml file

   query {
     project(fullPath: "your/project") {
       ciPipelineCreationInputs(ref: "main", failOnCacheMiss: true) {
         name
         default
         required
       }
     }
   }

   • Returns error: "Failed to retrieve pipeline inputs from cache." on cache miss

3. Test with feature flag disabled:

   Feature.disable(:ci_pipeline_inputs_reactive_cache)

   • Query should return inputs directly without caching (original behavior)

MR acceptance checklist

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