Backport: fix (provider/xai): handle new reasoning text chunk parts

[vercel-ai-sdk]

This is an automated backport of #12362 to the release-v5.0 branch. FYI @shaper
This backport has conflicts that need to be resolved manually.

git cherry-pick output

CONFLICT (file location): examples/ai-functions/src/stream-text/xai-responses-grok-code-fast-1.ts added in 8b3e72dec (fix (provider/xai): handle new reasoning text chunk parts (#12362)) inside a directory that was renamed in HEAD, suggesting it should perhaps be moved to examples/ai-core/src/stream-text/xai-responses-grok-code-fast-1.ts.
Auto-merging packages/xai/src/responses/xai-responses-api.ts
Auto-merging packages/xai/src/responses/xai-responses-language-model.test.ts
CONFLICT (content): Merge conflict in packages/xai/src/responses/xai-responses-language-model.test.ts
Auto-merging packages/xai/src/responses/xai-responses-language-model.ts
error: could not apply 8b3e72dec... fix (provider/xai): handle new reasoning text chunk parts (#12362)
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"

[vercel]

Additional Suggestion:

The response.reasoning_summary_part.added handler emits a reasoning-start event but does not register the item in activeReasoning, causing a duplicate reasoning-start to be emitted when response.output_item.done is later received for the same reasoning item.