[PD]: Support incremental transfer for mooncake transfer engine (#24257)

Co-authored-by: Shangming Cai <csmthu@gmail.com>

test/registered/unit/server_args/test_server_args.py

@@ -63,6 +63,28 @@ class TestLoadBalanceMethod(unittest.TestCase):
             str(context.exception),
         )
 
+    def test_pd_decode_radix_cache_allows_mooncake(self):
+        server_args = ServerArgs(
+            model_path="dummy",
+            disaggregation_mode="decode",
+            disaggregation_decode_enable_radix_cache=True,
+            disaggregation_transfer_backend="mooncake",
+        )
+
+        self.assertFalse(server_args.disable_radix_cache)
+
+    def test_pd_decode_radix_cache_rejects_unknown_backend(self):
+        with self.assertRaises(ValueError) as context:
+            ServerArgs(
+                model_path="dummy",
+                disaggregation_mode="decode",
+                disaggregation_decode_enable_radix_cache=True,
+                disaggregation_transfer_backend="fake",
+            )
+
+        self.assertIn("('nixl', 'mooncake')", str(context.exception))
+        self.assertIn("'fake'", str(context.exception))
+
 
 class TestPortArgs(unittest.TestCase):
     @patch("sglang.srt.server_args.get_free_port")