[vlm] adjust max length for special tokens (#37342)

* update

* apply suggestion

* fix tests for main branch

* remove unused logger

* add special tokens in tests

* nit

* fix more tests

* fix test

* pg also

tests/models/gemma3/test_processing_gemma3.py

@@ -124,3 +124,28 @@ class Gemma3ProcessorTest(ProcessorTesterMixin, unittest.TestCase):
         # base image + 4 crops
         self.assertEqual(len(inputs[self.images_input_name]), 5)
         self.assertEqual(len(inputs[self.text_input_name][0]), 67)
+
+    def test_special_mm_token_truncation(self):
+        """Tests that special vision tokens do not get truncated when `truncation=True` is set."""
+
+        processor = self.get_processor()
+
+        input_str = self.prepare_text_inputs(batch_size=2, modality="image")
+        image_input = self.prepare_image_inputs(batch_size=2)
+        _ = processor(
+            text=input_str,
+            images=image_input,
+            return_tensors="pt",
+            truncation=None,
+            padding=True,
+        )
+
+        with self.assertRaises(ValueError):
+            _ = processor(
+                text=input_str,
+                images=image_input,
+                return_tensors="pt",
+                truncation=True,
+                padding=True,
+                max_length=5,
+            )