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 use of /v1/chat/completions and disabling /v1/completions. xAI.supportsCompletions() now returns false so reasoning models work without endpoint errors.

^{Written for commit 30dd91b. Summary will update on new commits.}

[cubic-dev-ai]

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