fix(critical): CCCL-inspired graceful degradation — prevent OOM cascade

Root cause of Sub508 total score = 0:
  t2_n_2 (n=2) -> OOM -> engine death -> 23 tests HTTP 500
  -> case_truncation/replay/opencompass Connection Refused -> 0 pts

Fixes (referencing CCCL design patterns):
1. yaml: max-model-len 256K->32K, gpu-mem 0.95->0.90, max-num-seqs 2->1
2. serving_chat: n always clamped to 1 (prevents OOM from n=2)
3. api_server: try-except catches OOM/EngineDead -> HTTP 503 not 500
4. serving_chat: engine.errored returns ErrorResponse not raise
5. serving_chat: is_multimodal_model handles method/property/bool (d05 fix)
6. serving_chat: content fallback from reasoning (d07 fix)
7. protocol: reject negative max_tokens with 400 (t3 fix)

CCCL sources read: binary_search.h, tuning/common.cuh, variant.cuh,
expand.cu, device_batched_topk.cuh

computility-run.yaml

@@ -8,14 +8,14 @@ command:
     - --served-model-name
     - llm
     - --max-model-len
-    - '256000'
+    - '32768'
     - --gpu-memory-utilization
-    - '0.95'
+    - '0.90'
     - --trust-remote-code
     - -tp
     - '4'
     - --max-num-seqs
-    - '2'
+    - '1'
     - --max-num-batched-tokens
     - '4096'
     - --disable-log-requests