feat(issue-116): add shared validation helpers

[mxllv]

Summary

Added a small shared validation module so token-core and escrow paths can reuse the same checks for metadata inputs, amount semantics, ledger expiration boundaries, and frozen-account guards.

Changes

• Added src/validation.rs with shared helpers for non-empty strings, decimal bounds, positive/non-negative amounts, expiration-ledger validation, and frozen-account checks
• Updated metadata.rs to use the shared metadata validation helpers
• Updated allowance.rs to use shared checks for non-negative allowance amounts and current-or-future expiration ledgers
• Updated contract.rs, balance.rs, and escrow.rs to use shared positive-amount and frozen-account validation where those checks were duplicated or missing
• Files affected: veritixpay/contract/token/src/lib.rs, veritixpay/contract/token/src/validation.rs, veritixpay/contract/token/src/metadata.rs, veritixpay/contract/token/src/allowance.rs, veritixpay/contract/token/src/balance.rs, veritixpay/contract/token/src/escrow.rs, veritixpay/contract/token/src/contract.rs

Validation

• cd veritixpay/contract/token && cargo +stable build
• cd veritixpay/contract/token && cargo +stable test
• Results: build passed; tests passed (19 passed, 16 ignored)

Mismatch Handling (if applicable)

• The current repo already had metadata validation in one place, frozen-account checks repeated in several others, and amount/expiration validation either duplicated or missing depending on the path
• Added only the smallest viable scaffold: one focused validation.rs module and the direct call-site updates needed in token-core and escrow paths
• The solution was kept minimal by avoiding broader storage, auth, or arithmetic refactors

Notes

• approve continues to allow 0 as a revoke path, while negative allowances are now rejected through the shared helper
• This branch intentionally centralizes validation only; it does not change event schemas or storage layout

Closes #116

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