Executes the currency exchange (Buy or Sell) between the account's base currency and a supported foreign currency.

Functional Flow:

• Buy Currency: The base currency sub-account is debited (including any applicable fees), and the target foreign currency sub-account is credited.
• Sell Currency: The foreign currency sub-account is debited, and the base currency sub-account is credited.

Business Rules:

• Charges are applied exclusively to BUY operations and are debited from the base currency sub-account.
• Transaction sub-types are recorded as "Buy/Sell Currency" for clear ledger identification.
• If a rate was previously locked, the fxLockId should be provided in the request body to ensure the transaction uses the agreed rate.
• All conversions and fees are processed through the base currency sub-account.