Change Plan Mode to allow multiple negative and boundary checks per item,
while keeping happy path as a single check.
Schema change:
- checks.negative: Check -> Vec<Check> (>=1 required)
- checks.boundary: Check -> Vec<Check> (>=1 required)
- checks.happy: Check (unchanged, single)
This better reflects real-world tasks where there are often multiple
error conditions and edge cases worth tracking.
Changes:
- Update Checks struct to use Vec<Check> for negative/boundary
- Update validation to require at least 1 of each
- Update prompts and tool definitions with new array syntax
- Add 4 new tests for multi-check scenarios
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