fix: honor LangGraph chat model kwargs

[Kovbo]

Summary

Closes #474

Fixes ART's LangGraph init_chat_model wrapper so it no longer silently ignores the model argument, provider kwargs, or timeout configuration. Also clarifies that RULER's default judge calls OpenAI unless callers choose a different LiteLLM judge model.

Changes

• Forward init_chat_model(..., **kwargs) to ChatOpenAI, including provider-level timeout, temperature, and other supported kwargs.
• Add invoke_timeout for ART's outer asyncio.wait_for timeout, keeping it separate from provider request timeout.
• Honor an explicit string model name while still defaulting to the active ART rollout context model.
• Allow callers to pass a prebuilt LangChain chat model instance, such as ChatOllama, for non-OpenAI providers.
• Reject unsupported string model_provider values instead of silently constructing ChatOpenAI anyway.
• Preserve configured kwargs and invoke timeout across bind_tools(), with_structured_output(), and with_config().
• Add unit coverage for argument forwarding, custom model wrapping, timeout behavior, and LangGraph binding/config behavior.
• Document that RULER's default judge is OpenAI-backed and should be overridden for local/custom judge backends.

Test plan

• uv run ruff format --check src/art/rewards/ruler.py src/art/langgraph/llm_wrapper.py tests/unit/test_langgraph_llm_wrapper.py
• uv run ruff check src/art/rewards/ruler.py src/art/langgraph/llm_wrapper.py tests/unit/test_langgraph_llm_wrapper.py
• uv run ty check src/art/rewards/ruler.py src/art/langgraph/llm_wrapper.py tests/unit/test_langgraph_llm_wrapper.py
• uv run pytest tests/unit/test_langgraph_llm_wrapper.py tests/unit/test_ruler_metrics.py -q
• uv run pytest tests/unit/test_langgraph_llm_wrapper.py tests/unit/test_ruler_metrics.py tests/unit/test_model_openai_client_costs.py -q
• uv run pytest tests/unit -q --ignore=tests/unit/test_pipeline_trainer_local_backend.py --ignore=tests/unit/test_tinker_renderers.py --ignore=tests/unit/test_moe_routing_replay.py (251 passed, 7 skipped)

Full uv run pytest tests/unit -q is blocked in this local environment by missing optional dependencies (megatron, tinker_cookbook); the unignored failures were limited to tests requiring those packages.