[router] Improve SGLang chat render parity (#39133)

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>
Co-authored-by: Shangming Cai <csmthu@gmail.com>

experimental/sgl-router/tests/scripts/generate_array_content_fixture.py

@@ -0,0 +1,50 @@
+"""Regenerate the array-only template regression using SGLang's content processor.
+
+Run: PYTHONPATH=../../python python tests/scripts/generate_array_content_fixture.py
+"""
+
+import json
+from pathlib import Path
+
+from transformers import PreTrainedTokenizerFast
+
+from sglang.srt.parser.jinja_template_utils import (
+    detect_jinja_template_content_format,
+    process_content_for_template_format,
+)
+
+ROOT = Path(__file__).resolve().parents[1] / "fixtures"
+TEMPLATE = (
+    "{% for message in messages %}{{ message.role }}:"
+    "{% for part in message.content %}"
+    "{% if part.type == 'text' %}{{ part.text }}"
+    "{% elif part.type == 'image' %}<image>{% endif %}"
+    "{% endfor %};{% endfor %}"
+    "{% if add_generation_prompt %}assistant:{% endif %}"
+)
+
+
+def main():
+    tokenizer = PreTrainedTokenizerFast(
+        tokenizer_file=str(ROOT / "tiny_tokenizer.json")
+    )
+    tokenizer.chat_template = TEMPLATE
+    content_format = detect_jinja_template_content_format(TEMPLATE)
+    assert content_format == "openai"
+    cases = []
+    for content in ["hello", [{"type": "text", "text": "hello"}]]:
+        message = process_content_for_template_format(
+            {"role": "user", "content": content}, content_format, [], [], [], []
+        )
+        ids = tokenizer.apply_chat_template(
+            [message], return_dict=False, add_generation_prompt=True
+        )
+        cases.append(json.dumps({"content": content, "engine_token_ids": ids}))
+    cases_json = ",\n".join(cases)
+    (ROOT / "array_content_rendering.json").write_text(
+        f'{{"chat_template": {json.dumps(TEMPLATE)}, "cases": [\n{cases_json}\n]}}\n'
+    )
+
+
+if __name__ == "__main__":
+    main()