agentx: keep prefix caching ON in kimik2.7 none cell

The none case previously passed --no-enable-prefix-caching, which disables the
on-GPU prefix cache entirely, not just DRAM offload. That crippled the no-offload
baseline (no reuse even within HBM) and made lmcache look artificially good. Leave
prefix caching at vLLM's default (ON) so none is an honest GPU-only-KV baseline,
apples-to-apples vs lmcache. Matches the kimik2.5 / dsv4 agentic recipes.

Author: hyukjlee

benchmarks/single_node/agentic/kimik2.7_fp4_mi355x.sh

@@ -130,7 +130,12 @@ fi
 
 case "$OFFLOAD_MODE" in
     none)
-        OFFLOAD_ARGS=(--no-enable-prefix-caching)
+        # GPU-only KV, no DRAM offload. Leave prefix caching at vLLM's
+        # default (ON) so this is an honest no-offload baseline that still
+        # reuses shared prefixes on-GPU — apples-to-apples vs the lmcache
+        # cell, which extends that same reuse into DRAM. (Matches the
+        # kimik2.5 / dsv4 agentic recipes.)
+        PREFIX_CACHE_ARGS=(--enable-prefix-caching)
         ;;
     lmcache)
         unset VLLM_USE_SIMPLE_KV_OFFLOAD