* fix(studio): handle multimodal list content in inference text paths
Studio receives chat message content in two shapes: the legacy string
form, and the OpenAI multimodal list form
([{"type": "text", "text": ...}, {"type": "image_url", ...}]).
Several string-only paths called .strip()/re.sub()/f-string interpolation
on content directly, raising "'list' object has no attribute 'replace'"
for vision models (issue #4383), or rendering the list repr into the
prompt for the manual chat-template formatters.
Add core/inference/message_content.py with content_to_text(), a pure
helper (no heavy imports) that returns strings unchanged and joins the
text parts of a list while dropping image/audio parts. Apply it at every
string-only content site: _generate_vision_response, the audio user-text
extraction, format_chat_prompt, and the llama3/mistral/chatml/alpaca/
generic template formatters. The plain-string path is a no-op, so
existing behavior is unchanged.
Adds tests/test_message_content.py covering str/None/list/tuple,
multimodal drop, multi-part join and empty-part skipping.
* Tighten code comments (no logic change)
* studio: join multimodal text parts with newline for llama.cpp parity
llama.cpp joins multiple text content parts with a newline (common/chat.cpp),
so match that in content_to_text instead of a single space.
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Co-authored-by: Daniel Han <michaelhan2050@gmail.com>
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