Show error alert when CI variables cache is unavailable

What does this MR do and why?

This MR improves the user experience when CI/CD variables fail to load on the "Run Pipeline" page due to backend infrastructure issues.

Problem: When Sidekiq is saturated or experiencing high load, the ReactiveCache job for fetching CI config variables gets queued but never executes. Currently, the GraphQL query returns null, which the frontend treats as "no variables exist" - creating a silent failure where users cannot distinguish between legitimate empty state and an error state.

Solution: This MR adds proper error handling across the stack:

• Backend service now raises an error when cache is not ready
• GraphQL resolver propagates this error to the frontend
• Frontend displays a user-friendly error message

Closes #535560

Screenshots or screen recordings

Before

GraphQL `ciConfigVariables` query response

{
    "data": {
        "project": {
            "id": "gid://gitlab/Project/22",
            "ciConfigVariables": null,
            "__typename": "Project"
        }
    }
}

After

GraphQL `ciConfigVariables` query response

{
    "errors": [
        {
            "message": "Failed to retrieve CI configuration variables from cache.",
            "locations": [
                {
                    "line": 4,
                    "column": 5
                }
            ],
            "path": [
                "project",
                "ciConfigVariables"
            ]
        }
    ],
    "data": {
        "project": {
            "id": "gid://gitlab/Project/22",
            "ciConfigVariables": null,
            "__typename": "Project"
        }
    }
}

How to set up and validate locally

Prerequisites

1. Ensure GDK is running
2. Create a test project with a .gitlab-ci.yml that has prefilled variables:

variables:
  DEPLOY_ENVIRONMENT:
    value: "staging"
    description: "The environment to deploy to (staging, production)"
  
  VERSION:
    value: "1.0.0"
    description: "Version number for the deployment"

stages:
  - test

test_job:
  stage: test
  script:
    - echo "Deploying to $DEPLOY_ENVIRONMENT"
    - echo "Version $VERSION"

How to setup and validate locally

1. Stop Sidekiq workers:

   gdk stop rails-background-jobs

2. Clear the cache for your project:

   gdk rails console

   In the console:

   Rails.cache.clear
   exit

3. Navigate to the Run Pipeline page:

   • Go to your project
   • Click Build → Pipelines → Run pipeline

4. Verify the error is displayed:

   • You should see an alert banner with the message: "Failed to retrieve CI configuration variables from cache."
   • Previously, the page would show no variables without any indication of an error

MR acceptance checklist

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

• I have evaluated the MR acceptance checklist for this merge request.