add per-variant endpoint concurrency with least-loaded dispatch

[hallerite]

Description

When multiple evals target the same endpoint (e.g., 8 vLLM nodes serving the same model), each eval creates its own semaphore independently. This means there's no shared concurrency control, and blind round-robin ignores node load, which causes head-of-line blocking when one node is slower.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Test improvement

Testing

• All existing tests pass when running uv run pytest locally.
• New tests have been added to cover the changes

Checklist

• My code follows the style guidelines of this project as outlined in AGENTS.md
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Additional Notes