More resilient eval (#32257)

Bubbles up rate limit information so that we can retry after a certain
duration if needed higher up in the stack.

Also caps the number of concurrent evals running at once to also help.

Release Notes:

- N/A

.github/workflows/unit_evals.yml

@@ -62,7 +62,7 @@ jobs:
 
       - name: Run unit evals
         shell: bash -euxo pipefail {0}
-        run: cargo nextest run --workspace --no-fail-fast --features eval --no-capture -E 'test(::eval_)' --test-threads 1
+        run: cargo nextest run --workspace --no-fail-fast --features eval --no-capture -E 'test(::eval_)'
         env:
           ANTHROPIC_API_KEY: ${{ secrets.ANTHROPIC_API_KEY }}