[AMD] qwen3.5-fp8-mi355x-sglang-agentic-hicache: fix runner to mi355x

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: seungrokj

.github/configs/amd-master.yaml

@@ -2610,7 +2610,7 @@ qwen3.5-fp8-mi355x-sglang-agentic-hicache:
   image: lmsysorg/sglang-rocm:v0.5.12.post1-rocm720-mi35x-20260531
   model: Qwen/Qwen3.5-397B-A17B-FP8
   model-prefix: qwen3.5
-  runner: b300
+  runner: mi355x
   precision: fp8
   framework: sglang
   multinode: false