fix(disagg): allow fake transfer with decode DCP (#35409)

Signed-off-by: Alexandre Milesi <milesial@users.noreply.github.com>

python/sglang/srt/arg_groups/pd_disaggregation_hook.py

@@ -35,10 +35,16 @@ def handle_pd_disaggregation(server_args: ServerArgs) -> None:
         )
 
     if server_args.disaggregation_mode == "decode" and server_args.dcp_size > 1:
-        if server_args.disaggregation_transfer_backend not in ("mooncake", "nixl"):
+        # Fake transfer moves no KV and is only used for synthetic decode
+        # benchmarks, so it does not need the DCP relayout from Mooncake/NIXL.
+        if server_args.disaggregation_transfer_backend not in (
+            "mooncake",
+            "nixl",
+            "fake",
+        ):
             raise ValueError(
                 "PD decode DCP requires --disaggregation-transfer-backend "
-                "mooncake or nixl, got "
+                "mooncake, nixl, or fake for synthetic benchmarking, got "
                 f"{server_args.disaggregation_transfer_backend!r}."
             )
         if server_args.disaggregation_decode_enable_radix_cache: