fix: cancelOrder now returns transaction hash like cancelOrders

[Sertug17]

Summary

cancelOrder computed transactionHash internally but never returned it,
leaving callers with no way to obtain it. cancelOrders the plural
counterpart has always returned the hash.

Changes

• src/sdk/cancellation.ts: add return transactionHash + Promise<string> return type
• src/sdk/base.ts: add Promise<string> return type + @returns JSDoc tag

Proof

// BEFORE — hash computed and discarded
// Transact and get the transaction hash   ← comment already said "get"
const transactionHash = await this.cancelSeaportOrders({...})
await this.context.confirmTransaction(transactionHash, ...)
// no return

// AFTER
return transactionHash  // ← one line fix

This is a non-breaking additive change existing callers that ignore the
return value are unaffected.

Fixes #1981