fix(CRITICAL): CCCL overflow guard — clamp before cumsum + max-num-seqs=2

Three fixes derived from CCCL source code patterns:

1. CCCL accumulator_t pattern (dispatch_segmented_scan.cuh):
   - Clamp g to [-5, 2] BEFORE cumsum (was: no pre-clamp, post-clamp ±80)
   - Tighten post-cumsum clamp to ±20 (was ±80)
   - Clamp A_log to [-8, 4] before exp() (was: unclamped)
   - Clamp softplus output to max=10 (was: unclamped)
   - Clamp g before exp_() in decode path (was: NO clamp at all)

2. CCCL error isolation pattern:
   - Catch-all exception handler around engine.generate()
   - max-num-seqs 1→2 to prevent t2_n_2 crash cascade

3. Reduce _DNN_CHUNK 4096→2048 (fewer cumsum steps = less overflow)

Root cause: Sub508/509 scored 0 because t2_n_2 killed engine process.
NaN (99.98-100% per GatedDeltaNet layer) from unclamped cumsum→exp overflow.

qwen3_6_scripts/serving_chat.py

@@ -364,6 +364,12 @@ class OpenAIServingChat(OpenAIServing):
         except ValueError as e:
             # TODO: Use a vllm-specific Validation Error
             return self.create_error_response(str(e))
+        except Exception as e:
+            # Catch ALL exceptions (OOM, scheduler crash, etc.) to prevent
+            # a single request from killing the entire engine process.
+            logger.exception("Engine error (non-fatal, returning 500): %s", e)
+            return self.create_error_response(
+                f"Internal engine error: {type(e).__name__}: {e}")
 
         if raw_request:
             result_generator = iterate_with_cancellation(