feat(xai): support base_url alias and XAI_API_BASE env var (#35790)

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`

libs/partners/xai/langchain_xai/chat_models.py

@@ -7,7 +7,7 @@ from typing import TYPE_CHECKING, Any, Literal, TypeAlias, cast
 
 import openai
 from langchain_core.messages import AIMessageChunk
-from langchain_core.utils import secret_from_env
+from langchain_core.utils import from_env, secret_from_env
 from langchain_openai.chat_models.base import BaseChatOpenAI
 from pydantic import BaseModel, ConfigDict, Field, SecretStr, model_validator
 from typing_extensions import Self
@@ -397,6 +397,7 @@ class ChatXAI(BaseChatOpenAI):  # type: ignore[override]
 
     model_name: str = Field(default="grok-4", alias="model")
     """Model name to use."""
+
     xai_api_key: SecretStr | None = Field(
         alias="api_key",
         default_factory=secret_from_env("XAI_API_KEY", default=None),
@@ -405,8 +406,16 @@ class ChatXAI(BaseChatOpenAI):  # type: ignore[override]
 
     Automatically read from env variable `XAI_API_KEY` if not provided.
     """
-    xai_api_base: str = Field(default="https://api.x.ai/v1/")
-    """Base URL path for API requests."""
+
+    xai_api_base: str = Field(
+        alias="base_url",
+        default_factory=from_env("XAI_API_BASE", default="https://api.x.ai/v1/"),
+    )
+    """Base URL path for API requests.
+
+    Automatically read from env variable `XAI_API_BASE` if not provided.
+    """
+
     search_parameters: dict[str, Any] | None = None
     """**Deprecated.** Use web search tools instead: