Backport: fix(xai): negative text_output_tokens for xAI reasoning models

[ghost]

This is an automated backport of #11536 to the release-v5.0 branch. FYI @copilot
This backport has conflicts that need to be resolved manually.

git cherry-pick output

CONFLICT (modify/delete): packages/xai/src/convert-xai-chat-usage.test.ts deleted in HEAD and modified in 77012ef8b (fix(xai): negative text_output_tokens for xAI reasoning models (#11536)).  Version 77012ef8b (fix(xai): negative text_output_tokens for xAI reasoning models (#11536)) of packages/xai/src/convert-xai-chat-usage.test.ts left in tree.
CONFLICT (modify/delete): packages/xai/src/convert-xai-chat-usage.ts deleted in HEAD and modified in 77012ef8b (fix(xai): negative text_output_tokens for xAI reasoning models (#11536)).  Version 77012ef8b (fix(xai): negative text_output_tokens for xAI reasoning models (#11536)) of packages/xai/src/convert-xai-chat-usage.ts left in tree.
Auto-merging packages/xai/src/xai-chat-language-model.test.ts
CONFLICT (content): Merge conflict in packages/xai/src/xai-chat-language-model.test.ts
error: could not apply 77012ef8b... fix(xai): negative text_output_tokens for xAI reasoning models (#11536)
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".
hint: Disable this message with "git config set advice.mergeConflict false"

[lgrammel]

backporting does not make sense, this is a languagemodelv3 thing