Backport: Backport: fix (provider/xai): add response.incomplete and response.failed streaming event handling

[vercel-ai-sdk]

This is an automated backport of #14221 to the release-v5.0 branch. FYI @vercel-ai-sdk[bot]
This backport has conflicts that need to be resolved manually.

git cherry-pick output

Auto-merging packages/xai/src/responses/map-xai-responses-finish-reason.ts
Auto-merging packages/xai/src/responses/xai-responses-api.ts
CONFLICT (content): Merge conflict in packages/xai/src/responses/xai-responses-api.ts
Auto-merging packages/xai/src/responses/xai-responses-language-model.test.ts
Auto-merging packages/xai/src/responses/xai-responses-language-model.ts
CONFLICT (content): Merge conflict in packages/xai/src/responses/xai-responses-language-model.ts
error: could not apply c1cc97f15... Backport: fix (provider/xai): add response.incomplete and response.failed streaming event handling (#14221)
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"