convert test_deterministic into unit tests

[skyzh]

Motivation

Part of #10278

Modifications

The patch converts the deterministic test into unit tests so that it's easier to detect issues in CI and avoid regression.

• add n_start parameter so that we can start with 10+ size batch, saving some time
• avoid supplying sampling seed as None as it would cause sglang server to stop (a bug to be fixed in the server?)
• add return value for test_deterministic: an array of unique responses for each prompt

Accuracy Tests

Benchmarking and Profiling

Checklist

• Format your code according to the Format code with pre-commit.
• Add unit tests according to the Run and add unit tests.
• Update documentation according to Write documentations.
• Provide accuracy and speed benchmark results according to Test the accuracy and Benchmark the speed.

[merrymercy]

the test code is wrong https://github.com/sgl-project/sglang/actions/runs/18264854581/job/52013897713?pr=11095#step:5:1635

[Fridge003]

the test code is wrong https://github.com/sgl-project/sglang/actions/runs/18264854581/job/52013897713?pr=11095#step:5:1635

Fixed

[Fridge003]

LGTM

[skyzh]

Thanks for following up :) Did not have my personal laptop with me last week 😂