Migrate CI workflows to Blacksmith runners

[haasonsaas]

Summary

• Switch test job from ubuntu-latest to blacksmith-4vcpu-ubuntu-2404
• Matches evalops/llm-gateway#94

Test plan

• CI passes on Blacksmith runners

[cursor]

PR Summary

Low Risk
Low risk since this only changes the CI runner environment, but it may surface OS/image-specific dependency or tooling differences in the test job.

Overview
Switches the GitHub Actions ci workflow test job runner from ubuntu-latest to blacksmith-4vcpu-ubuntu-2404 to run CI on Blacksmith-hosted Ubuntu 24.04.

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