fix(types): validate individual ratio legs in validateSplitRatioSum

[neverm1ndthat]

Fixes #1012

Summary

validateSplitRatioSum() in splitExecutor.ts now validates each recipient ratio individually before checking their aggregate sum.

The Bug

validateSplitRatioSum() only verified that |sum - 1| < tolerance. This allowed invalid inputs to pass:

• [-1, 2] - sums to 1 but contains a negative ratio and a ratio > 1
• NaN / Infinity legs - Math.abs(NaN - 1) > tolerance always evaluates to false, so NaN sums silently pass

The Fix

1. Iterate each recipient with a declared ratio and reject any that is not finite or outside [0, 1]
2. Also guard the aggregate sum with Number.isFinite before the tolerance check
3. Adds a new SplitRatioLegError that surfaces which recipient has the bad ratio and why

Testing

• All 10 existing tests in test/splitExecutorRatio.test.ts continue to pass
• New SplitRatioLegError class properly extends StellarSplitError with code SPLIT_RATIO_LEG_INVALID

[neverm1ndthat]

Hi @Kingsman-99! This PR addresses Issue #1012 - validateSplitRatioSum now rejects individual ratio legs that are not finite or outside [0,1], not just the aggregate sum. This catches cases like [-1, 2] that sum to 1 but are semantically invalid. PTAL!