Address PR #19533 review: guard test on jinja2cpp; document empty tools default

- test/CMakeLists.txt: exclude test_jinja_chat_formatter.cpp from the test
  binary when jinja2cpp isn't built (mirrors the runner CMake guard), so
  building tests without the chat_template subdir doesn't fail to link with
  undefined JinjaChatFormatter symbols (review #4).
- jinja_chat_formatter.cpp: document that the empty `tools` list is
  intentionally falsy so the normalized no-tools template path renders
  (review #1).

Author: seyeong-han

extension/llm/runner/jinja_chat_formatter.cpp

@@ -359,6 +359,11 @@ std::string JinjaChatFormatter::formatConversation(
   params["add_generation_prompt"] = conversation.add_generation_prompt;
   // Provide vLLM/HuggingFace-style defaults that templates often reference.
   // Templates that don't use these will simply ignore them.
+  // `tools` is intentionally an empty list (not null): Jinja2Cpp, like Python,
+  // treats an empty list as falsy, so `{% if tools %}` and the normalized
+  // `tools is not none` -> `tools` checks render the "no tools" path when none
+  // are supplied. See normalizeTemplate() and the UniversalJinjaToolsAware
+  // test.
   params["tools"] = jinja2::ValuesList();
   params["tool_choice"] = jinja2::Value();
   params["date_string"] = conversation.date_string;

extension/llm/runner/test/CMakeLists.txt

@@ -28,6 +28,14 @@ set(_test_srcs
     test_wav_loader.cpp
 )
 
+# test_jinja_chat_formatter.cpp exercises JinjaChatFormatter, which is only
+# compiled into extension_llm_runner when jinja2cpp is available (see
+# ../CMakeLists.txt). Drop the test source when jinja2cpp is not built so the
+# test binary does not fail to link with undefined JinjaChatFormatter symbols.
+if(NOT TARGET jinja2cpp)
+  list(FILTER _test_srcs EXCLUDE REGEX "test_jinja_chat_formatter\\.cpp$")
+endif()
+
 # Add LSan stub for Apple platforms
 if(APPLE)
   list(APPEND _test_srcs lsan_stub.cpp)