fix: edit mode throwing error for xai provider

[uinstinct]

Description

force xai to always use /v1/chat/completions because reasoning models do not support the /v1/completions endpoint

fixes #10963

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Summary by cubic

Fixes edit mode errors with the xAI provider by forcing the chat completions API and disabling the completions endpoint. xAI.supportsCompletions() now returns false so reasoning models run without endpoint errors.

^{Written for commit ed94d7d. Summary will update on new commits.}

[cubic-dev-ai]

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