[Bug]: Add tests for all parallel sampling parameter combinations

Verify that we have tests that explicitly exercise parallel sampling (n>1 request sampling parameter) for all of the following combinations:

• Via AsyncLLM.generate, via LLMEngine add_request() / step()
• For output_kind equal to each of CUMULATIVE, DELTA, FINAL_ONLY

Ideally a test for each of AsyncLLM and LLMEngine in the appropriate files, with output_kind parameterized.

Additionally we can still test LLM.generate() but this enforces FINAL_ONLY. There is already a test for this one here, but I'm not sure about the other cases.

@sethkimmel3 has reported that the LLMEngine + CUMULATIVE combination is not working properly, this should be exposed and fixed if so.