Persist and surface prefilled variables from PEP

What does this MR do and why?

Variables defined as a hash with description can be prefilled when running pipeline manually.

This change collects and persists these variables from pipeline execution policy configs and surfaces them together with the project variables.

References

• Documentation: https://docs.gitlab.com/ci/pipelines/#prefill-variables-in-manual-pipelines
• Discussion about the priority of this feature: #527021 (comment 2600030410)

Screenshots or screen recordings

PEP variables showing up when running a pipeline manually:

Recording of applying the variable values with multiple PEPs:

CleanShot_2025-10-07_at_17.05.53

How to set up and validate locally

1. Create a project

2. Create a new pipeline execution policy and define some prefill variables, declaring description attribute. Don't forget to allowlist the variables that are expected to receive user input:

   • pep.yml:

     variables:
       POLICY_VAR:
         value: 'a'
         description: 'Policy var'
         options:
         - 'a'
         - 'b'
         - 'c'
       POLICY_MODE:
         value: 'strict'
         description: 'Mode of the policy.'
         options:
         - 'strict'
         - 'loose'
       FREETEXT_VAR:
         description: 'free text var'
       OTHER_VAR: 'not a prefill variable'
     
     policy-job:
       script:
         - echo "POLICY_VAR = $POLICY_VAR"
         - echo "POLICY_MODE = $POLICY_MODE"
         - echo "FREETEXT_VAR = $FREETEXT_VAR"
         - echo "OTHER_VAR = $OTHER_VAR"
         - echo "DISALLOWED_VAR = $DISALLOWED_VAR"

   • policy.yml:

     ---
     pipeline_execution_policy:
     - name: Prefilled variables
       description: ''
       enabled: true
       pipeline_config_strategy: inject_policy
       content:
         include:
         - project: gitlab-org/pep/prefilled-variables
           file: policy.yml
       skip_ci:
         allowed: false
       variables_override:
         allowed: false
         exceptions:
         - POLICY_VAR
         - POLICY_MODE
         - FREETEXT_VAR
     

3. In the project, run a new pipeline manually

4. Verify that policy variables show up in the list of prefilled variables

MR acceptance checklist

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Related to #527021