fix(hicache): support legacy SGLang CP context

[ketor]

Problem

xb01 production rolled back the v2.24.3 client after the first Spinfield-managed GLM-5.2 worker crashed in _resolve_parallel_coordinates: its older SGLang ParallelContext exposes attn_cp_size/rank but no attn_dcp_size/rank.

LWS stopped after the first unready worker, limiting impact to one of 17 workers.

Fix

• preserve the legacy attn_cp_* axis as PCP
• treat a completely absent independent DCP axis as (1, 0)
• do not copy the same legacy CP coordinate into both PCP and DCP, which would duplicate key dimensions and alter replica-writer election
• retain strict validation when only part of an axis is present

Verification

• added a legacy ParallelContext regression that lacks both DCP fields
• python3 integration/hicache/tests/test_sg_width_namespace.py: 17/17 passed

The broader unittest discovery also reaches the same 17 passing tests; its unrelated test_client_stats module cannot import locally because this workstation lacks pytest. CI supplies the pinned test environment.