feat: Add xAiGrok Chat Model node and credentials

[riascho]

Summary

This PR adds a new node for the xAI Grok Chat Model and credentials.

To-do:

• update docs reference in node once we have it

Related Linear tickets, Github issues, and Community forum posts

• https://linear.app/n8n/issue/NODE-2487/create-grokxai-node

Review / Merge checklist

• PR title and summary are descriptive. (conventions)
• Docs updated or follow-up ticket created.
• Tests included.
• PR Labeled with release/backport (if the PR is an urgent fix that needs to be backported)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 22 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...ain/nodes/llms/LmChatXAiGrok/LmChatXAiGrok.node.ts	0.00%	15 Missing ⚠️
.../nodes-langchain/credentials/XAiApi.credentials.ts	0.00%	7 Missing ⚠️

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