add regression test (#35834)

Signed-off-by: hallerite <git@hallerite.com>

tests/entrypoints/openai/test_tokenization_vlm.py

+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+"""
+Regression test: ``/tokenize`` must expand image placeholders for VLM models.
+
+Fixed by PR #34560 ("Move InputPreprocessor into Renderer (2/2)").
+Before that change, ``/tokenize`` returned ~26 tokens for a message with an
+image instead of the expected 1451.  Confirmed broken on 0.15.1 and 0.16.0.
+"""
+
+import json
+
+import pytest
+import requests
+
+from ...utils import RemoteOpenAIServer
+
+MODEL_NAME = "Qwen/Qwen2.5-VL-3B-Instruct"
+
+
+@pytest.fixture(scope="module")
+def server():
+    args = [
+        "--dtype",
+        "bfloat16",
+        "--max-model-len",
+        "4096",
+        "--max-num-seqs",
+        "5",
+        "--enforce-eager",
+        "--limit-mm-per-prompt",
+        json.dumps({"image": 1}),
+    ]
+    with RemoteOpenAIServer(MODEL_NAME, args) as remote_server:
+        yield remote_server
+
+
+def test_tokenize_chat_expands_image_placeholders(
+    server: RemoteOpenAIServer,
+    local_asset_server,
+):
+    image_url = local_asset_server.url_for("stop_sign.jpg")
+    messages = [
+        {
+            "role": "user",
+            "content": [
+                {"type": "image_url", "image_url": {"url": image_url}},
+                {"type": "text", "text": "Describe this image."},
+            ],
+        }
+    ]
+
+    response = requests.post(
+        server.url_for("tokenize"),
+        json={"model": MODEL_NAME, "messages": messages},
+    )
+    response.raise_for_status()
+
+    # stop_sign.jpg (1300x876) produces 1451 tokens after expansion.
+    # Without expansion the count would be ~26 (text + one placeholder).
+    assert response.json()["count"] == 1451