Loosen the xAI integration tests to handle two recent provider
behaviors: `web_search` responses may now include block types beyond the
core trio, and streaming aggregation under `output_version="v1"` does
not produce a `tool_call` content block (tool calls are only available
on `.tool_calls`).
Add `base_url` alias and `XAI_API_BASE` env variable support to
`ChatXAI.xai_api_base`, aligning the xAI integration with the pattern
used across other partner packages (OpenAI, Groq, Fireworks, etc.).
Previously the base URL was a plain string field with no alias or
env-var lookup, making it inconsistent with the rest of the ecosystem
and harder to configure in deployment environments.
## Changes
- Add `alias="base_url"` and `default_factory=from_env("XAI_API_BASE",
default="https://api.x.ai/v1/")` to `ChatXAI.xai_api_base`, matching the
convention in `langchain_openai`, `langchain_groq`, and
`langchain_fireworks`
- Test if models support forcing tool calls via `tool_choice`. If they
do, they should support
- `"any"` to specify any tool
- the tool name as a string to force calling a particular tool
- Add `tool_choice` to signature of `BaseChatModel.bind_tools` in core
- Deprecate `tool_choice_value` in standard tests in favor of a boolean
`has_tool_choice`
Will follow up with PRs in external repos (tested in AWS and Google
already).
