Backport: [v6.0] fix(provider/xai): extract reasoning text from content in responses doGenerate

[github-actions]

This is an automated backport of #16798 to the release-v5.0 branch. FYI @gr2m
This backport has conflicts that need to be resolved manually.

git cherry-pick output

CONFLICT (file location): examples/ai-functions/src/generate-text/xai/responses-reasoning.ts added in 19eece65b8 ([v6.0] fix(provider/xai): extract reasoning text from content in responses doGenerate (#16798)) inside a directory that was renamed in HEAD, suggesting it should perhaps be moved to examples/ai-core/src/generate-text/responses-reasoning.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
CONFLICT (content): Merge conflict in packages/xai/src/responses/xai-responses-language-model.ts
error: could not apply 19eece65b8... [v6.0] fix(provider/xai): extract reasoning text from content in responses doGenerate (#16798)
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