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
• Python test matrix runs correctly

[cursor]

PR Summary

Low Risk
Low risk workflow-only change that alters the CI execution environment; failures are most likely due to runner compatibility rather than code behavior.

Overview
Switches the CI test job runner from ubuntu-latest to blacksmith-4vcpu-ubuntu-2404, keeping the existing Python matrix and steps unchanged.

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