[Bugfix][KV Connector][NIXL] Support matching DCP layouts with PCP

[LucasWilkinson]

Note: this PR intentionally supports matching producer and consumer DCP sizes with PCP disabled on the consumer; unequal-DCP remapping and decoder-side PCP remain out of scope.

Purpose

Add DCP-aware NIXL P/D transfer support that composes with prefill PCP without changing attention or token-interleaving behavior.

This change:

• uses explicit (pp_rank, tp_rank, pcp_rank) handshake keys and (pp_rank, tp_rank, dcp_rank) remote-agent keys;
• routes each consumer to producer workers with the same DCP rank while preserving the existing TP transfer plan;
• rejects mismatched producer and consumer DCP sizes;
• rejects PCP on KV consumers, allowing consumer fan-in to use the direct tp_rank % dcp_size mapping;
• publishes one PCP producer replica per distinct DCP shard and counts completion only from those published replicas;
• preserves one-copy MLA pulls while notifying the other producer workers involved in the request.

The connector remains layout-opaque: it selects matching workers and copies KV blocks. PCP/DCP attention chunking, empty attention shards, decoder-side PCP, and unequal-DCP token-position remapping are outside this PR.

Related work and duplicate check

Open-PR searches for NIXL PCP/DCP and context-parallel NIXL changes found #38433, #45340, and this draft; no other open PR combines the reduced equal-DCP transfer path with PCP replica ownership.

• feat(nixl,dcp): Supports MLA DCP for PD disaggregation with nixl connector #38433 is the generalized unequal-DCP implementation. This PR is intentionally smaller: it accepts only matching DCP layouts and adds PCP publication/completion behavior.
• [KV Connector][NIXL][Mooncake] CP-scaled scheduler block accounting for PD disaggregation with aligned DCP/PCP #45340 changes scheduler-side CP-scaled block accounting while leaving transfer-worker routing unchanged.

Test plan and results

.venv/bin/python -m pytest tests/v1/kv_connector/unit/test_nixl_connector.py tests/v1/kv_connector/unit/test_nixl_push_connector.py -k "not abort_timeout_on_prefiller and not register_kv_caches and not reqs_to_send_deadline_rebased_to_worker_clock" -q
# 102 passed, 7 deselected

.venv/bin/python -m pytest tests/v1/kv_connector/unit/test_handshake_pp_aggregation.py tests/v1/kv_connector/unit/test_nixl_connector.py::TestNixlHandshake::test_consumer_rejects_pcp tests/v1/kv_connector/unit/test_nixl_connector.py::TestNixlHandshake::test_pcp_producer_publishes_one_replica_per_dcp_rank tests/v1/kv_connector/unit/test_nixl_connector.py::TestNixlHandshake::test_pcp_producer_waits_only_for_published_replicas tests/v1/kv_connector/unit/test_nixl_connector.py::TestNixlHandshake::test_prefill_tp_size_greater_than_decode_tp_size tests/v1/kv_connector/unit/test_nixl_connector.py::TestNixlHandshake::test_prefill_tp_size_greater_than_decode_tp_size_mla tests/v1/kv_connector/unit/test_nixl_push_connector.py::TestPushPipelineParallel tests/v1/kv_connector/unit/test_nixl_push_connector.py::TestPushWriterMlaReplication tests/v1/kv_connector/unit/test_transfer_topology_sharded.py -q
# 17 passed

.venv/bin/pre-commit run --files $(git diff --name-only upstream/main)
# passed

A final-branch four-GPU TP1+PCP2+DCP2+EP2 prefiller to TP2+DCP2 decoder run was attempted. NIXL initialized and model loading completed, but current main deadlocked before the first transfer during PCP×DCP attention warmup: the non-empty PCP worker entered the DCP collective while its empty-context peer skipped it. Because this is a core PCP×DCP attention issue rather than connector behavior, this PR does not add a failing integration job for that topology.

Earlier development on a larger superset branch completed both 1,319-example DeepSeek-V2-Lite-Chat GSM8K evaluations for TP1+PCP2+DCP2 to TP2+DCP2 and TP2+PCP2+DCP4 to TP4+DCP4 within the required accuracy band. Those runs also contained separate attention and MRV2 fixes that are not in this connector PR, so they are not final-branch validation.

AI assistance disclosure

OpenAI Codex assisted with investigation, implementation, testing, and drafting this PR.

Reviewed by the human submitter

[mergify]

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

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, @LucasWilkinson.

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

[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.

[mergify]

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

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

[LucasWilkinson]

Superseded by #52779, which contains only the PCP producer-replica portion on current main. The DCP routing/configuration portion is handled separately by #50611.