[Bugfix][Structured Output] Mask grammar-invalid implicit reasoning ends (#61574)

Signed-off-by: Maroon Ayoub <mayoub@redhat.com>
Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Robert Shaw <114415538+robertgshaw2-redhat@users.noreply.github.com>

vllm/parser/engine/parser_engine.py

@@ -678,6 +678,11 @@ class ParserEngine(Parser):
     def reasoning_end_token_ids(self) -> frozenset[int]:
         return self._reasoning_end_token_ids
 
+    @property
+    def implicit_reasoning_end_token_ids(self) -> frozenset[int]:
+        """Reasoning-end tokens that are also content, e.g. a tool-call start."""
+        return self._reasoning_end_token_ids - {self._reasoning_end_token_id}
+
     def find_reasoning_end(self, token_ids: Sequence[int]) -> ReasoningEnd | None:
         """First reasoning end in `token_ids`, and if its token is content."""
         end_ids = self._reasoning_end_token_ids