fix: stop normalizeMoneyAmount from silently truncating sub-cent Stellar amounts (Closes #439)

[s6pa1rta3n-lab]

Summary

Fixes #439 by making normalizeMoneyAmount precision-aware instead of hard-coding 2 decimal places.

Key Changes

• Preserves fractional digits up to 7 decimal places (Stellar stroops) by default, preventing sub-cent amounts like '0.0000001' and '0.0000009' from silently truncating to '0.00'.
• Supports an explicit scale parameter (0-7) and NormalizeMoneyAmountOptions with optional round: boolean flag.
• Strips leading zeros from the integer component (e.g., '0075.5' -> '75.50').
• Validates target scale bounds and throws RangeError if scale < 0, scale > 7, or !Number.isInteger(scale).
• Uses exact string and BigInt arithmetic to avoid IEEE-754 floating-point precision issues during rounding.
• Comprehensive test coverage in tests/money-amount.test.ts for sub-cent amounts, custom scale, half-up rounding, RangeError boundaries, and compatibility with validateMoneyAmount.

Verification

• npx vitest run tests/money-amount.test.ts tests/request-validation.test.ts: Passed (22/22 tests).
• npx vitest run tests/models.test.ts tests/moneyamount-passthrough.test.ts tests/public-api-types.test.ts tests/validation.test.ts tests/payment-validation.test.ts: Passed (63/63 tests).
• npx prettier --check src/models/common.ts tests/money-amount.test.ts: Passed.
• npx eslint src/models/common.ts tests/money-amount.test.ts: Passed (0 errors, 0 warnings).
• npx tsup: Build succeeded (ESM, CJS, and DTS bundles emitted).

Payout Routing

• EVM (Base/Arbitrum/Polygon/ETH): 0xF46C9F6d70C50BF81ef3588AB523a90a594a2F89
• Stellar: GCL6OXAMLD75BMTINA6EMRUDWK5THQUSHMYNLSNBCJAPZJHNYJTUNIBC