fix(lmstudio): handle system messages regardless of position

[BlueX888]

Summary

LMStudioTypeAdapter.format_chat_model_input raised ValueError: Unsupported role: system for any system message that was not the first message of the chat, including chats that contain a second system message. A system message in first position with an empty-string content was silently dropped instead of being passed along.

Why

The Chat input documents that each message's role "can be 'user', 'assistant', or 'system'" with no positional constraint (src/outlines/inputs.py:82). The Gemini adapter already follows that contract: it collects system messages regardless of their position and joins them (src/outlines/models/gemini.py:87-95, introduced in #1967).

The LMStudio adapter instead only lifted a system message when it was the first message, and built the chat with LMSChat(system_prompt) if system_prompt else LMSChat(). Any other system message fell through to the role check and raised ValueError: Unsupported role: system (src/outlines/models/lmstudio.py:122 on main). The if system_prompt test also treated an empty-string system message as absent.

Changes

• src/outlines/models/lmstudio.py: format_chat_model_input now adds each system message at the position it appears in, via the SDK's Chat.add_system_prompt. Consecutive system messages are joined with "\n", because lmstudio 1.5.0 raises LMStudioRuntimeError: Multi-part or consecutive system prompts are not supported. when two system prompts are added back to back. An empty-string system message is preserved rather than dropped.
• tests/models/test_lmstudio_type_adapter.py: regression tests for a system message in the middle of the chat, consecutive system messages, an empty-string system message, and a trailing system message.

Testing

Run locally on Python 3.12 with lmstudio 1.5.0:

pytest tests/models/test_lmstudio_type_adapter.py -k "system_not_first or multiple_system or empty_system or trailing_system"
# 4 passed, 17 deselected
# (on main, all 4 fail: ValueError: Unsupported role: system, or the
#  empty-string system message is missing from the resulting chat)

pytest tests/models/test_lmstudio_type_adapter.py tests/models/test_lmstudio.py
# 42 passed, 4 skipped

coverage run --branch --source=outlines -m pytest tests/models/test_lmstudio_type_adapter.py tests/models/test_lmstudio.py
coverage report
# src/outlines/models/lmstudio.py    149 stmts, 58 branches, 0 missed   100%

pre-commit run --all-files
# check for merge conflicts ......... Passed
# debug statements (python) ......... Passed
# fix end of files .................. Passed
# trim trailing whitespace .......... Passed
# mypy .............................. Passed
# ruff .............................. Passed

Checklist:

• We should be able to understand what the PR does from its title only;
• There is a high-level description of the changes;
• If I add a new feature, there is an issue discussing it already; (bug fix, not a new feature)
• There are links to all the relevant issues, discussions and PRs; (no existing issue covers this; the collect-anywhere system-message behavior was introduced for Gemini in fix(gemini): pass the system message as a system instruction #1967)
• The branch is rebased on the latest main commit;
• Commit messages follow these guidelines;
• One commit per logical change;
• The code respects the current naming conventions;
• Docstrings follow the numpy style guide;
• pre-commit is installed and configured on your machine, and you ran it before opening the PR;
• There are tests covering the changes;
• The documentation is up-to-date;