sponsorship.ts: Fix sponsored account fee not attributed to sponsor in event payload

[Kingsman-99]

Description

src/sponsorship.ts emits a SponsorshipUsed event when a sponsored transaction is submitted, but the event payload's feeSource field is set to the submitter's account rather than the sponsor account. This causes incorrect fee attribution in downstream analytics.

Acceptance Criteria

• SponsorshipUsed event feeSource field is set to the sponsor account ID
• A test asserts the correct feeSource in the emitted event for a sponsored transaction
• Non-sponsored transactions continue to set feeSource to the submitter
• Unit tests pass

Context

• Target file: src/sponsorship.ts
• The sponsor account ID is available in the transaction envelope; no additional Horizon calls are needed