[V1] [KVConnector] Fix MultiprocExecutor worker output aggregation

[sdavidbd]

Purpose

Fix an issue in MultiprocExecutor where the finished_sending and finished_recving fields in aggregated worker outputs were not correctly updated. This could wrongly propagate some requests as finished even when certain workers had not reported them yet.

This PR also adds comprehensive unit tests for _aggregate_workers_output and _async_aggregate_workers_output to ensure correct behavior in various scenarios.

Test Plan

Run the new unit tests:

$ pytest -s -v tests/v1/executor/test_multiproc_executor.py

Test Result

All tests pass. Example output:

============================================================================================ test session starts =============================================================================================
platform linux -- Python 3.12.9, pytest-8.3.5, pluggy-1.6.0 -- /home/pliops/miniconda3/envs/dbd-env/bin/python3.12
cachedir: .pytest_cache
rootdir: /home/pliops/workspace/davidb/git/vllm-dbd
configfile: pyproject.toml
plugins: anyio-4.9.0
collected 2 items

tests/v1/executor/test_multiproc_executor.py::test_aggregate_workers_output PASSED
tests/v1/executor/test_multiproc_executor.py::test_async_aggregate_workers_output PASSED

============================================================================================= 2 passed in 0.30s ==============================================================================================

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[njhill]

Thanks @sdavidbd, and especially for adding these missing tests.