feeBumpBuilder.ts: Fix fee-bump envelope not setting the correct source account

[Kingsman-99]

Description

src/feeBumpBuilder.ts constructs fee-bump transaction envelopes, but the source account field is set from the inner transaction's source rather than the fee-bump payer account passed as a parameter. This causes transaction submission to fail with an authorization error.

Acceptance Criteria

• The fee-bump envelope's source account is set from the feeSource parameter passed to buildFeeBump()
• The inner transaction source account is unchanged
• A test compares the built envelope's source account to the provided feeSource
• Unit tests pass

Context

• Target file: src/feeBumpBuilder.ts
• This is a one-line fix; tests are the majority of the work