Add request concurrency test coverage

Syed Saif Ali Alvirequested to merge
gitlab-community/gitlab-org/gitlab-runner:syed/gitlab-runner/i-38735/concurrency-test-coverage into main

What does this MR do?

Add request concurrency test coverage

Integration tests for GitLab runner concurrency limit enforcement. These tests validate that runner job limits are properly enforced under various concurrent access patterns.

What are the relevant issue numbers?

#38735

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