#402 - test(multisig_split_refund): add extreme-value boundary tests for checked arithmetic

[qa-eden]

Closes #402

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Summary

multisig_split_refund already delegates all arithmetic to split_round_nearest, which uses checked_mul, checked_add, and checked_sub throughout. The BPS pair addition is also guarded with checked_add. No production-code changes are required.

What was missing was test coverage proving that extreme operands (i128::MAX, i128::MIN, negatives) return typed errors rather than panicking or wrapping.

Changes

multisig_split_refund_tests.rs — new dedicated test file, 12 tests across 6 sections:

• Auth / source-state guards — unauthorized caller and unlocked state rejected before any arithmetic runs
• total_amount boundary guards — i128::MIN, negatives, and zero all return Error::InvalidAmount
• Overflow cases — i128::MAX with every non-trivial BPS pair returns Error::InvalidAmount; u32::MAX BPS pair returns Error::InvalidRatio
• Extreme amounts that succeed — i128::MAX with 0-bps client completes correctly; i64::MAX 50/50 split sums exactly to the input
• Valid-amount regression — 50/50, 70/30, odd rounding, extreme BPS pin exact outputs unchanged
• Event emission — success emits exactly one splitref event; all failure paths emit none

test.rs — adds the #[path] mod declaration following the existing pattern used by multisig_admin_override_refund_tests and multisig_transfer_admin_tests.

Testing

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