Fix fee bump source account

[kingsrichie79-cmyk]

Closes #700

Description:

Description

This PR resolves an issue where src/feeBumpBuilder.ts constructed fee-bump transaction envelopes using the inner transaction's source account instead of the explicitly provided fee-bump payer account. This incorrect assignment was causing transaction submissions to fail with an authorization error.

Root Cause & Fix

When passing a string to @stellar/stellar-sdk's TransactionBuilder.buildFeeBumpTransaction(), the source account was not being properly respected in the resulting envelope, falling back to the inner transaction's source.

To fix this, we now explicitly instantiate a Keypair from the provided feeSource public key using Keypair.fromPublicKey(feeSource). This forces the SDK to use the correct feeSourceAccount, ensuring the fee-bump payer account is set properly while leaving the inner transaction's source untouched.

Acceptance Criteria Met

• The fee-bump envelope's source account is correctly set from the feeSource parameter passed to buildFeeBump().
• The inner transaction source account remains unchanged.
• Added unit tests comparing the built envelope's .feeSource property to the provided feeSource string.
• All unit tests pass successfully.

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