[Offload] Deduplicate lockstep MLA pending job cleanup

[procr1337]

This fixes a KeyError bug when using native KV offload on v20 GLM-5.2 image

Opened against local-inference-lab#197

Lockstep MLA groups share physical block IDs, so a transfer job may contain the same block ID once per logical group. Iterate the deduplicated set of block IDs in _remove_pending_job() so each (block, job) pair is only removed once from the set-backed pending-job index, instead of crashing with KeyError on the second occurrence.

Fixes a KeyError in _remove_pending_job() observed when KV cache offloading (--kv-offloading-size) is enabled while serving GLM-5.2 (MLA + decode context parallelism), reproduced by
offload-fix-glm52/probe_gpu_cache.py.

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