Summary

When MLA is enabled, the world_size passed to LMCache is divided by tp_size (since all TP ranks share the same KV cache in MLA models). However, the read lock count during lookup still needs the original TP size to acquire the correct number of locks for all workers that will independently retrieve the same cached chunks.

This PR adds a tp_size parameter to LMCacheMPSchedulerAdapter and propagates it through IPCCacheEngineKey so the LMCache server can use it for multi-reader locking.

Changes

vllm side (this PR)

• create_scheduler_adapter: extracts tp_size from parallel_config and passes it
• _adapter_accepts_tp_size(): inspect-based compatibility check so newer vLLM works with older LMCache
• Fallback LMCacheMPSchedulerAdapter: accepts tp_size and propagates it in key creation

LMCache side (companion PR: LMCache/LMCache#2697)

• IPCCacheEngineKey: adds tp_size: int = 1 field (backward compatible default)
• LMCacheMPSchedulerAdapter: accepts tp_size parameter
• server.py lookup(): uses key.tp_size instead of key.world_size for extra_readers

Backward Compatibility

• New vllm + old LMCache: _adapter_accepts_tp_size() detects the old signature and skips passing tp_size
• Old vllm + new LMCache: tp_size defaults to 1 in both the adapter and the key
• New vllm + new LMCache: full TP-aware multi-reader locking enabled