fix(harness): guard against invalid resuming a session vs continuing a turn

[felixarntz]

Summary

This PR improves/fixes a few related harness issues:

• HarnessAgent had a continueTurn that was returning a StreamTextResult, but it missed the corresponding equivalent that returns a GenerateTextResult. This PR replaces the method with continueStream and continueGenerate.
  • These methods are responsible for continuing a session where there's a mid-flight suspended/interrupted turn (e.g. due to requesting tool approval, or due to manual suspension via suspendTurn).
• HarnessAgent now more cleanly separates "resume session state" and "continue turn state", with only the latter being able to continue pending tool approvals (because by definition tool approval requests lead to an interrupted turn).
• HarnessAgent now guards against incorrect method invocation: when a session is resumed with state that indicates that there's a mid-flight turn, calling stream or generate will error. Accordingly, if the resume state indicates there is no mid-flight turn, calling continueStream or continueGenerate will error.

Manual Verification

Run the existing examples.

Checklist

• All commits are signed (PRs with unsigned commits cannot be merged)
• Tests have been added / updated (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)
• A patch changeset for relevant packages has been added (for bug fixes / features - run pnpm changeset in the project root)
• I have reviewed this pull request (self-review)

[github-actions]

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Package	Version
ai	7.0.0-canary.172 github npm
@ai-sdk/alibaba	2.0.0-canary.59 github npm
@ai-sdk/amazon-bedrock	5.0.0-canary.84 github npm
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