Centralize amount validation by wiring amount_validation into create_contract and deposit_funds

[mikewheeleer]

Description

contracts/escrow/src/amount_validation.rs provides validate_milestone_amounts, validate_deposit_amount, and stroop-precision helpers, but create_contract and deposit_funds in lib.rs re-implement their own ad-hoc loops with safe_add_amounts. Consolidate to a single validation path to remove drift between the two checks.

Requirements and context

• Scoped to TalentTrust escrow Soroban contract (contracts/escrow).
• Route create_contract milestone validation through validate_milestone_amounts and deposit_funds through validate_deposit_amount, mapping AmountValidationError variants to the corresponding EscrowError codes.
• Preserve existing error semantics (InvalidMilestoneAmount, PotentialOverflow, DepositWouldExceedTotal).
• Invariant: validation behavior is identical before and after, with one source of truth.
• Must be secure, tested, and documented.

Suggested execution

• Fork the repo and create a branch:
  • git checkout -b enhancement/centralize-amount-validation
• Implement changes:
  • contracts/escrow/src/lib.rs
  • Tests: contracts/escrow/src/test/input_sanitization_amounts.rs
  • Docs: docs/escrow/milestone-validation.md
  • Include rustdoc/NatSpec-style doc comments on public functions
  • Validate security assumptions (auth, overflow, fail-closed state machine, storage TTL, fee accounting)

Test and commit

• Run tests: cargo test
• Cover edge cases (unauthorized callers, double release/refund, expired approvals, fee rounding, paused state)
• Include test output and security notes in the PR

Example commit message

refactor(escrow): route amounts through amount_validation

Guidelines

• Minimum 95% test coverage on new/changed code
• Clear documentation
• Timeframe: 96 hours from assignment

[wavyboy-build]

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