docs(huggingface): add chat usage guide and regression tests for init_chat_model

[Accidental-MVP]

Description

Add offline unit tests documenting the current Hugging Face initialization bug and a short docs page for HF chat usage.

Tests (unit, offline)

• Add libs/langchain/tests/unit_tests/chat_models/test_init_chat_model_hf.py.
• Fake transformers.pipeline and langchain_huggingface.chat_models.huggingface.ChatHuggingFace via monkeypatch.setitem(sys.modules, ...).
• Extend importlib.util.find_spec so nested modules are treated as installed.
• Pass task="text-generation" explicitly to avoid default drift.
• Assert current behavior by expecting a TypeError (since ChatHuggingFace is constructed without llm):
  • calling init_chat_model(..., model_provider="huggingface") raises
  • same with max_tokens present (documenting the max_tokens → max_new_tokens expectation)
  • same with timeout / max_retries present (they shouldn't be sent to the pipeline)
• Tests are offline and deterministic; they verify today's failure mode so we have a clear regression harness when the fix lands.

Docs

• Add docs/docs/integrations/chat/huggingface.mdx following the required template sections:
  • Overview, Setup, Instantiation, Invocation, Chaining, API reference.
• Option 1 (works today): transformers.pipeline → ChatHuggingFace(llm=...).
• Option 2 (after fix): init_chat_model(..., model_provider="huggingface") usage, clearly gated as future.
• Note: install (langchain-huggingface, transformers) and prefer max_new_tokens over max_tokens.

Issue

Refs #28226
Related: #33167, #32941

Dependencies

None.

Test plan

Local (Windows/PowerShell), using the repo's uv test group:

py -m pipx run uv sync --group test
py -m pipx run uv pip install --group test langchain-huggingface transformers
py -m pipx run uv run --group test pytest .\tests\unit_tests\chat_models\test_init_chat_model_hf.py -q

Expected: 3 passed (they assert the current TypeError), no hard failures.

Lint & CI

• Formatted & fixed with ruff locally:
  • py -m pipx run ruff check libs/langchain/tests/unit_tests/chat_models/test_init_chat_model_hf.py --fix
  • py -m pipx run ruff format libs/langchain/tests/unit_tests/chat_models/test_init_chat_model_hf.py
• Will address any CI feedback (ruff/pre-commit, docs checks) if raised.

Notes for reviewers

• Tests intentionally lock in the current failure mode (explicit pytest.raises(TypeError)) so we can flip them to assert the fixed behavior once the upstream change merges (pipeline created and passed as llm=...).
• Happy to:
  • update tests to assert the fixed path after merge, or
  • submit a follow-up PR removing the error assertions.
• Fine with moving/renaming the test file or adjusting assertions to match house style.

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Project	Deployment	Preview	Comments	Updated (UTC)
langchain	Ready	Preview	Comment	Oct 2, 2025 2:36am

[mdrxy]

We've moved docs away from this repo - see https://github.com/langchain-ai/docs

[Accidental-MVP]

thanks! I’ll move the docs to langchain-ai/docs and keep this PR tests-only.