fix(mp): propagate remote handler failures to clients

[voipmonitor]

Problem

A blocking or synchronous MP handler exception was only logged by the server. No response was sent, so the client MessagingFuture remained pending forever and vLLM requests could remain in WAITING_FOR_REMOTE_KVS.

Change

• add exception completion to MessagingFuture
• return a bounded, versioned RPC error envelope from sync and blocking handlers
• surface remote failures as RemoteHandlerError on the client
• keep ZMQ access on the server loop by queueing callback responses
• return an explicit error for unregistered request types

The wire change is error-only: successful response frames are unchanged. Error payloads contain the exception type and a message capped at 4096 bytes; server tracebacks stay in server logs.

Validation

• Ruff check and format pass
• 33 MQ/future tests pass; 13 CUDA-only tests skip in the test container
• sync exception completes the future and the same polling loop processes a later healthy request
• blocking exception completes the future instead of timing out

Reported by Florian Bernd during GLM-5.2 LMCache testing. This PR is intentionally the first layer; retrieve recovery and storage policy changes remain separate.

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Run ID: d5073e92-67ac-4f29-8348-843196fce638

📥 Commits

Reviewing files that changed from the base of the PR and between 9cebd40 and 7b7583a.

📒 Files selected for processing (5)

• lmcache/v1/multiprocess/futures.py
• lmcache/v1/multiprocess/mq.py
• tests/v1/multiprocess/test_futures.py
• tests/v1/multiprocess/test_mq.py
• tests/v1/multiprocess/test_mq_handler_helpers.py

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