[Bugfix] Handle parenthesized Gemma4 tool calls (#53657)

Signed-off-by: Taneem Ibrahim <taneem.ibrahim@gmail.com>

vllm/parser/gemma4.py

@@ -285,7 +285,7 @@ def _parse_gemma4_array(arr_str: str, *, partial: bool = False) -> list:
 def _gemma4_arg_converter(raw_args: str, partial: bool) -> str:
     """Convert Gemma4 custom arg format to a JSON string."""
     text = raw_args.strip()
-    if text.endswith("}"):
+    if text.endswith("}") or text.endswith(")") and text.count("(") < text.count(")"):
         text = text[:-1]
 
     parsed = _parse_gemma4_args(text, partial=partial)
@@ -304,6 +304,7 @@ def gemma4_config() -> ParserEngineConfig:
             "TOOL_END": TOOL_CALL_END,
             "CALL_PREFIX": "call:",
             "OPEN_BRACE": "{",
+            "OPEN_PAREN": "(",
         },
         token_id_terminals={
             "THINK_START": CHANNEL_START,
@@ -351,6 +352,14 @@ def gemma4_config() -> ParserEngineConfig:
                 ParserState.TOOL_ARGS,
                 (),
             ),
+            (ParserState.TOOL_NAME, "OPEN_PAREN"): Transition(
+                ParserState.TOOL_ARGS,
+                (),
+            ),
+            (ParserState.TOOL_NAME, "TOOL_END"): Transition(
+                ParserState.CONTENT,
+                (EventType.TOOL_CALL_END,),
+            ),
             (ParserState.TOOL_ARGS, "TOOL_END"): Transition(
                 ParserState.CONTENT,
                 (EventType.TOOL_CALL_END,),