[Refactor] Keep chat template validation out of ServerArgs dispatcher (#33392)

Co-authored-by: Alex Nails <alex.nails@radixark.ai>

python/sglang/srt/server_args.py

@@ -3476,13 +3476,6 @@ class ServerArgs:
         # Validate transcription/ASR-specific server args.
         self._handle_asr_validation()
 
-        if self.default_chat_template_kwargs is not None and not isinstance(
-            self.default_chat_template_kwargs, dict
-        ):
-            raise ValueError(
-                "--default-chat-template-kwargs must decode to a JSON object"
-            )
-
         # Handle deprecated arguments.
         self._handle_deprecated_args()
 
@@ -8091,6 +8084,12 @@ class ServerArgs:
             )
 
     def _handle_other_validations(self):
+        if self.default_chat_template_kwargs is not None and not isinstance(
+            self.default_chat_template_kwargs, dict
+        ):
+            raise ValueError(
+                "--default-chat-template-kwargs must decode to a JSON object"
+            )
 
         # Handle optimistic prefill validation
         if (