fix(qwen3.5-fp4-mi355x-sglang-mtp): use scenarios/fixed-seq-len schema

The old 'seq-len-configs' field was renamed to 'scenarios.fixed-seq-len'
in the master config schema. Pydantic validation failed setup with
'Field required (scenarios)' + 'Extra inputs are not permitted (seq-len-configs)'.

Author: functionstackx

.github/configs/amd-master.yaml

@@ -375,17 +375,18 @@ qwen3.5-fp4-mi355x-sglang-mtp:
   precision: fp4
   framework: sglang
   multinode: false
-  seq-len-configs:
-  - isl: 1024
-    osl: 1024
-    search-space:
-    - { tp: 2, conc-start: 4, conc-end: 256, spec-decoding: mtp }
-    - { tp: 4, conc-start: 4, conc-end: 16, spec-decoding: mtp }
-  - isl: 8192
-    osl: 1024
-    search-space:
-    - { tp: 2, conc-start: 4, conc-end: 256, spec-decoding: mtp }
-    - { tp: 4, conc-start: 4, conc-end: 16, spec-decoding: mtp }
+  scenarios:
+    fixed-seq-len:
+    - isl: 1024
+      osl: 1024
+      search-space:
+      - { tp: 2, conc-start: 4, conc-end: 256, spec-decoding: mtp }
+      - { tp: 4, conc-start: 4, conc-end: 16, spec-decoding: mtp }
+    - isl: 8192
+      osl: 1024
+      search-space:
+      - { tp: 2, conc-start: 4, conc-end: 256, spec-decoding: mtp }
+      - { tp: 4, conc-start: 4, conc-end: 16, spec-decoding: mtp }
 
 qwen3.5-fp8-mi300x-sglang:
   image: lmsysorg/sglang:v0.5.10-rocm720-mi30x