Backport: [v6.0] fix(ai): enforce timeout.stepMs for the whole step in streamText

[github-actions]

This is an automated backport of #16801 to the release-v5.0 branch. FYI @gr2m
This backport has conflicts that need to be resolved manually.

git cherry-pick output

Auto-merging packages/ai/src/generate-text/stream-text.test.ts
CONFLICT (content): Merge conflict in packages/ai/src/generate-text/stream-text.test.ts
Auto-merging packages/ai/src/generate-text/stream-text.ts
CONFLICT (content): Merge conflict in packages/ai/src/generate-text/stream-text.ts
error: could not apply faaef7c83e... [v6.0] fix(ai): enforce timeout.stepMs for the whole step in streamText (#16801)
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".
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