[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/mistral3/test_processor_mistral3.py

@@ -290,3 +290,29 @@ class Mistral3ProcessorTest(ProcessorTesterMixin, unittest.TestCase):
         inputs_image = processor(text=prompt_string, images=image_inputs, return_tensors="pt", padding=True)
         self.assertIn("input_ids", inputs_image)
         self.assertTrue(len(inputs_image["input_ids"]) == 5)
+
+    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=3,
+            )