[Minor] Invert MRV1 specdec method fallback logic (#56899)

Signed-off-by: Nick Hill <nickhill123@gmail.com>

vllm/config/vllm.py

@@ -2829,16 +2829,16 @@ class VllmConfig:
             unsupported.append("pipeline parallelism with external_launcher")
 
         if speculative_config is not None:
-            # TODO: ngram / ngram_gpu are not supported by the v2 model runner yet
-            if speculative_config.method in ("ngram", "ngram_gpu"):
-                unsupported.append("ngram/ngram_gpu speculative decoding")
-            elif speculative_config.method not in (
-                "eagle",
-                "eagle3",
-                "mtp",
-                "dflash",
-                "dspark",
-                "extract_hidden_states",
+            if speculative_config.method in (
+                # https://github.com/vllm-project/vllm/pull/40704
+                "ngram",
+                "ngram_gpu",
+                # https://github.com/vllm-project/vllm/pull/43091
+                "draft_model",
+                "suffix",
+                "medusa",
+                "mlp_speculator",
+                "custom_class",
             ):
                 unsupported.append(f"speculative method '{speculative_config.method}'")