feat: add xAI integration

[raphaelchristi]

This pull request adds xAI (Grok) model integration to Langflow. The component supports dynamic model fetching from the xAI API, handles both chat and completion requests, and includes proper error handling. Built on the existing langchain-openai package with no additional dependencies required.

[edwinjosechittilappilly]

Few quick changes:

use MessageTextInput instead of StrInput

Also use variable name in Inputs: base_url instead of api_base.
To make it consistent for future changes.

Thanks for the PR.

[edwinjosechittilappilly]

Added Combobox to models.

[edwinjosechittilappilly]

LGTM

[edwinjosechittilappilly]

update tests ensure they pass.

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