[PD] Remove outdated backend whitelist for decode radix cache (#28238)

Signed-off-by: Shangming Cai <csmthu@gmail.com>

python/sglang/srt/arg_groups/pd_disaggregation_hook.py

@@ -31,16 +31,10 @@ def handle_pd_disaggregation(server_args: "ServerArgs") -> None:
                     "--disaggregation-decode-enable-radix-cache is incompatible "
                     "with --enable-hisparse"
                 )
-            if server_args.disaggregation_transfer_backend not in (
-                "nixl",
-                "mooncake",
-                "mori",
-            ):
+            if server_args.disaggregation_transfer_backend == "fake":
                 raise ValueError(
-                    "--disaggregation-decode-enable-radix-cache currently "
-                    "requires --disaggregation-transfer-backend in "
-                    "('nixl', 'mooncake', 'mori'), but got "
-                    f"{server_args.disaggregation_transfer_backend!r}"
+                    "--disaggregation-decode-enable-radix-cache is incompatible "
+                    "with --disaggregation-transfer-backend fake"
                 )
             if server_args.speculative_algorithm is not None:
                 raise ValueError(

python/sglang/srt/server_args.py

@@ -7387,7 +7387,7 @@ class ServerArgs:
         parser.add_argument(
             "--disaggregation-decode-enable-radix-cache",
             action="store_true",
-            help="Enable radix cache on decode server (PD mode). Caches KV prefixes to avoid redundant transfers. Requires --disaggregation-transfer-backend nixl, mooncake or mori and is incompatible with --enable-hisparse.",
+            help="Enable radix cache on decode server (PD mode). Caches KV prefixes to avoid redundant transfers. Incompatible with --enable-hisparse, speculative decoding, and --disaggregation-transfer-backend fake.",
         )
         parser.add_argument(
             "--disaggregation-decode-enable-offload-kvcache",