[Bugfix] Release ParentRequest state when an n>1 request is aborted

[UgaTheDev]

What is broken

OutputProcessor.abort_requests never drops the ParentRequest when an n>1
request is aborted by its external id.

An external id maps to the child internal ids, so the abort loop finds a
RequestState for every child and takes the if req_state is not None: branch.
That branch pops the child from self.request_states but never touches
self.parent_requests. The parent cleanup lives in the elif parent := self.parent_requests.get(request_id) branch, which only runs when req_state
is None, so it never fires here. _finish_request cannot clean up either,
since the child states are already gone.

The result is one ParentRequest left in self.parent_requests for the life of
the engine, holding its child_requests set and its output aggregator.

This needs n>1 plus an abort or a client disconnect. Normal completion and
n=1 aborts are unaffected, so it is a slow leak on servers that see parallel
sampling with cancellations, not a leak on every request.

Fix

In the if req_state is not None: branch, resolve the parent from the child's
own req_state.parent_req, discard the child, and pop the parent once its last
child is gone. This mirrors what _finish_request already does. The existing
elif branch is unchanged.

Test

Added test_abort_parallel_sampling_request in
tests/v1/engine/test_output_processor.py. It builds an n=3 request with a
ParentRequest and three children, aborts by internal ids and by external id,
and asserts parent_requests, request_states and external_req_ids are all
empty afterwards.

I could not run the test file on my machine: importing the test harness pulls in
vllm.v1.engine.llm_engine and the process dies with SIGBUS on macOS/CPU before
collection. I verified the fix with a standalone script driving OutputProcessor
directly with the same setup as the test. It fails on main with
{'request-0': <ParentRequest>} still in parent_requests and passes with this
change, for both the internal-id and external-id abort paths. CI should be the
judge of the pytest run.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

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[UgaTheDev]

Ready for review — bugfix to release ParentRequest state when an n>1 request is aborted, with a regression test; pre-commit clean locally. Could a maintainer add the ready label so CI can run? Thanks!

[UgaTheDev]

Friendly ping: pre-run-check here is failing only on the first-contributor gate (author has <4 merged PRs), not on the change itself. Could a maintainer add the ready label so the suite can run? Happy to address anything CI surfaces. Same applies to my other three open bugfix PRs (#51621, #51626, #51628) if you're willing.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @UgaTheDev.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @UgaTheDev.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork