Add short flag `-sc` for `--speculative-config` argument (#38380)

Co-authored-by: Claude <noreply@anthropic.com>

vllm/engine/arg_utils.py

@@ -1301,7 +1301,7 @@ class EngineArgs:
         # delay the Pydantic validation that comes with SpeculativeConfig.
         vllm_kwargs["speculative_config"]["type"] = optional_type(json.loads)
         vllm_group.add_argument(
-            "--speculative-config", **vllm_kwargs["speculative_config"]
+            "--speculative-config", "-sc", **vllm_kwargs["speculative_config"]
         )
         vllm_group.add_argument(
             "--kv-transfer-config", **vllm_kwargs["kv_transfer_config"]