Refactor TestRig to support configurable approval modes for behavioral evals #17168
Description
Replace the deprecated yolo boolean flag in TestRig with a flexible approvalMode property. This refactor aligns the test infrastructure with the core ApprovalMode logic, enabling tests to explicitly target modes like plan or default. Updates evals/test-helper.ts and existing integration tests to utilize this refined interface.
The TestRig should continue to default to YOLO approval mode.