Add instrumentation for inputs keyword usage tracking

What does this MR do and why?

• Add instrumentation for inputs keyword usage tracking
• Add tracking for the 'inputs:' keyword in pipeline configs and sets ups header level foundational instrumentation.

Validation Steps

1. Test basic inputs tracking:

   • Create a test project
   • Add this .gitlab-ci.yml:

     spec:
       inputs:
         environment:
           default: 'staging'
     ---
     test_job:
       script: echo "Environment is $[[ inputs.environment ]]"

   • Push and run pipeline
   • Check Rails console: Gitlab::InternalEvents.track_event('use_inputs_keyword_in_cicd_yaml', project: Project.find(PROJECT_ID))
   • Verify event was recorded in events table

2. Test input rules tracking:

   • Update .gitlab-ci.yml to:

     spec:
       inputs:
         environment:
           rules:
             - if: '$CI_COMMIT_REF_NAME == "main"'
               default: 'production'
             - default: 'staging'
     ---
     test_job:
       script: echo "Environment is $[[ inputs.environment ]]"

   • Push and run pipeline
   • Verify use_input_rules_keyword_in_cicd_yaml event was recorded

3. Test no false positives:

   • Create pipeline with regular .gitlab-ci.yml (no spec/inputs)
   • Verify neither event is tracked
   • Create pipeline with spec: but no inputs:
   • Verify neither event is tracked

MR acceptance checklist

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