Add failedJobsCount field to Pipeline GraphQL type

What does this MR do and why?

This MR adds a new failedBuildsCount field to the Ci::PipelineType GraphQL type to support the migration of the MR Pipelines page from REST to GraphQL.

As part of MR Pipelines migration to GraphQL (&14144) • Sahil Sharma • 18.7, we need to implement the Failed Jobs expandable widget that currently exists in the REST-based MR Pipelines page. This widget displays a count badge showing the number of failed jobs for each pipeline, allowing users to quickly identify and expand pipelines with failures.

The REST API already provides failed_builds_count via Ci::PipelineEntity, but this data was not available in GraphQL. This MR bridges that gap by exposing the same information through the GraphQL API.

References

Closes #581323

How to set up and validate locally

1. Enable the feature flag for GraphQL-based MR Pipelines:

   Feature.enable(:mr_pipelines_graphql)

2. Create a pipeline with multiple stages and failed builds across stages:

   Sample `.gitlab-ci.yml` configuration (click to expand)

    workflow:
      rules:
        - if: $CI_PIPELINE_SOURCE == "merge_request_event"
        - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH
        
    stages:
      - build
      - test
      - deploy
   
    build_job_1:
      stage: build
      script:
        - echo "Building..."
        - exit 0
   
    build_job_2:
      stage: build
      script:
        - echo "Building..."
        - exit 0  
   
    test_job_1:
      stage: test
      script:
        - echo "Testing..."
        - exit 1  # This will fail
   
    test_job_2:
      stage: test
      script:
        - echo "Testing..."
        - exit 1  # This will fail
   
    deploy_job:
      stage: deploy
      script:
        - echo "Deploying..."
        - exit 0
      when: manual
      allow_failure: true

3. Push this configuration to trigger a pipeline, or create one manually via the UI

4. Query the new field in GraphQL Explorer (/-/graphql-explorer):

   {
     project(fullPath: "your-namespace/your-project") {
       pipelines(last: 5) {
         nodes {
           iid
           status
           failedBuildsCount
         }
       }
     }
   }

5. Verify the failedBuildsCount returns 2 (matching the number of failed builds in the pipeline)

Database Review

Query Analysis

The failedBuildsCount field uses the existing Ci::Pipeline#limited_failed_builds association.

Raw SQL

SELECT
    "p_ci_builds ". "status ", "p_ci_builds ". "finished_at ", "p_ci_builds ". "created_at ", "p_ci_builds ". "updated_at ", "p_ci_builds ". "started_at ", "p_ci_builds ". "coverage ", "p_ci_builds ". "name ", "p_ci_builds ". "options ", "p_ci_builds ". "allow_failure ", "p_ci_builds ". "stage_idx ", "p_ci_builds ". "tag ", "p_ci_builds ". "ref ", "p_ci_builds ". "type ", "p_ci_builds ". "target_url ", "p_ci_builds ". "description ", "p_ci_builds ". "erased_at ", "p_ci_builds ". "artifacts_expire_at ", "p_ci_builds ". "environment ", "p_ci_builds ". "when ", "p_ci_builds ". "yaml_variables ", "p_ci_builds ". "queued_at ", "p_ci_builds ". "lock_version ", "p_ci_builds ". "coverage_regex ", "p_ci_builds ". "retried ", "p_ci_builds ". "protected ", "p_ci_builds ". "failure_reason ", "p_ci_builds ". "scheduled_at ", "p_ci_builds ". "token_encrypted ", "p_ci_builds ". "resource_group_id ", "p_ci_builds ". "waiting_for_resource_at ", "p_ci_builds ". "processed ", "p_ci_builds ". "scheduling_type ", "p_ci_builds ". "id ", "p_ci_builds ". "stage_id ", "p_ci_builds ". "partition_id ", "p_ci_builds ". "auto_canceled_by_partition_id ", "p_ci_builds ". "auto_canceled_by_id ", "p_ci_builds ". "commit_id ", "p_ci_builds ". "erased_by_id ", "p_ci_builds ". "project_id ", "p_ci_builds ". "runner_id ", "p_ci_builds ". "upstream_pipeline_id ", "p_ci_builds ". "user_id ", "p_ci_builds ". "execution_config_id ", "p_ci_builds ". "upstream_pipeline_partition_id ", "p_ci_builds ". "scoped_user_id ", "p_ci_builds ". "timeout ", "p_ci_builds ". "timeout_source ", "p_ci_builds ". "exit_code ", "p_ci_builds ". "debug_trace_enabled "
FROM
    "p_ci_builds "
WHERE
    "p_ci_builds ". "type " = 'Ci::Build'
    AND "p_ci_builds ". "commit_id " = 637
    AND "p_ci_builds ". "partition_id " = 100
    AND ("p_ci_builds ". "retried " = FALSE
        OR "p_ci_builds ". "retried " IS NULL)
    AND ("p_ci_builds ". "status " IN ('failed'))
LIMIT 101

Query Plan

Limit  (cost=0.00..0.00 rows=1 width=612) (actual time=0.003..0.003 rows=0 loops=1)
  ->  Seq Scan on ci_builds_100 p_ci_builds  (cost=0.00..0.00 rows=1 width=612) (actual time=0.003..0.003 rows=0 loops=1)
        Filter: (((NOT retried) OR (retried IS NULL)) AND ((type)::text = 'Ci::Build'::text) AND (commit_id = 637) AND (partition_id = 100) AND ((status)::text = 'failed'::text))
Planning:
  Buffers: shared hit=1085 read=43
Planning Time: 6.727 ms
Execution Time: 0.012 ms

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