fix(models): make the Mistral adapter work with mistralai 2.x and 3.x - #2073

Summary

from_mistral and every MistralTypeAdapter.format_input path imported Mistral, UserMessage, AssistantMessage and SystemMessage from the mistralai top level. mistralai 2.0.0 removed those top-level exports: the package became a namespace package whose client lives at mistralai.client and whose message classes live at mistralai.client.models. Since the mistral extra is declared without a version bound (pyproject.toml:48), a fresh pip install "outlines[mistral]" resolves mistralai 3.2.0, and every entry point (from_mistral, format_input, generate, stream) fails with an ImportError before any request is sent.

Why

• Root cause: src/outlines/models/mistral.py:81, :101, :120 and :545 import from the mistralai top level, which mistralai >= 2.0.0 no longer exports (checked against the 2.0.0, 2.10.1, 3.0.0 and 3.2.0 wheels: there is no mistralai/__init__.py; the client is at mistralai.client, the message classes at mistralai.client.models and SDKError at mistralai.client.errors).
• The runtime API the adapter calls (client.chat.complete / chat.stream / chat.complete_async / chat.stream_async) is unchanged in 3.2.0, so only the import paths need to move.
• The try-new-path/fallback-to-1.x pattern follows the existing convention in src/outlines/exceptions.py:298, which already resolves mistralai.client.errors with an ImportError fallback for older SDKs, so mistralai 1.x keeps working.

Fixes #2070

Changes

• src/outlines/models/mistral.py: import the client from mistralai.client and the message classes from mistralai.client.models, keeping the top-level imports as a fallback for mistralai 1.x (the uv.lock test environment still pins 1.9.11).
• tests/models/test_mistral.py, tests/models/test_mistral_type_adapter.py, tests/models/test_provider_exceptions.py: these imported from the removed surface themselves and failed at collection time on mistralai >= 2.0.0; updated with the same fallback.
• tests/models/test_mistral_sdk_compat.py (new): regression tests covering the four entry points (format_input with str/list/Chat, and from_mistral) against whichever mistralai release is installed.

Testing

On the unpatched tree with mistralai 3.2.0 (what a fresh outlines[mistral] install resolves), the new tests fail with the reported error:

$ python -m pytest tests/models/test_mistral_sdk_compat.py -m 'not api_call' -q
E   ImportError: cannot import name 'UserMessage' from 'mistralai' (unknown location)
src/outlines/models/mistral.py:81: ImportError
...
4 failed in 0.36s

On the patched tree:

$ python -m pytest tests/models/test_mistral_sdk_compat.py -m 'not api_call' -q
4 passed in 2.58s                                                    # mistralai 3.2.0

$ python -m pytest tests/models/test_mistral.py tests/models/test_mistral_type_adapter.py tests/models/test_mistral_sdk_compat.py tests/models/test_provider_exceptions.py -m 'not api_call' -q
94 passed, 18 deselected in 1.55s                                    # mistralai 3.2.0
94 passed, 18 deselected in 7.40s                                    # mistralai 1.9.11 (versions from uv.lock)

$ 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

The module suite passes both on mistralai 1.9.11 (the version pinned in uv.lock, used by CI) and on 3.2.0.

Checklist

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