feat: add error codes for group buy, mediation, and migration scenari…

[Stanley-Owoh]

Summary

• Added TypeScript SDK for frontend integration with the MarketX escrow contract
• Created comprehensive type definitions in sdk/types.ts mirroring all Rust contract types
• Updated sdk/error-codes.ts with missing error codes (160-174) for group buy, mediation, token circuit breaker, and migration scenarios
• Added sdk/index.ts as barrel export for easy importing
  Changes
• sdk/types.ts (new): TypeScript interfaces for Escrow, EscrowItem, Milestone, GroupBuy, MediationPhase, all contract events, and constants
• sdk/error-codes.ts: Added error codes 160-174 (GroupBuyDeadlineNotReached, AppealWindowNotClosed, MetadataAccessDenied, ZeroAddress, FeatureDisabled, InvalidAdminTransfer, MediationPhaseOpen, NoMediationPhase, MediationAlreadyConcluded, TokenPaused, Migration errors)
• sdk/index.ts (new): Barrel export for SDK
• contracts/marketx/src/errors.rs: Added missing Rust error variants
  Usage
  import { Escrow, getContractError } from './sdk';

Closes #207