fix(critical): match Sub168 proven config — max_model_len=100K, max_num_seqs=1, gpu_mem=0.9

Root cause analysis of Sub508 (41.2% score):
1. max_model_len=256000 → 100000 (Sub168 value)
   - Reduces KV cache preallocation by 2.56x
   - d01: should drop from 95.87s to ~8-10s
   - Frees GPU memory for stable inference

2. max_num_seqs=2 → 1
   - Eliminates t2_n_2 OOM crash that killed engine
   - Sub508 lost 23 tests + 881 replay to this single crash

3. gpu_memory_utilization=0.95 → 0.9 (matches Sub168 docker log)

4. serving_chat.py content fallback improved for d07

computility-run.yaml

@@ -8,14 +8,14 @@ command:
     - --served-model-name
     - llm
     - --max-model-len
-    - '256000'
+    - '100000'
     - --gpu-memory-utilization
-    - '0.95'
+    - '0.9'
     - --trust-remote-code
     - -tp
     - '4'
     - --max-num-seqs
-    - '2'
+    - '1'
     - --max-num-batched-tokens
     - '4096'
     - --disable-log-requests