[16] Add unit tests for `SponsorsService`

[EmeditWeb]

Problem

src/modules/sponsors/sponsors.service.ts has no unit tests. Pool stats aggregation, deposit XDR construction, and withdrawal validation are all unverified.

Context

Sponsors are the funding side of the protocol — any silent miscalculation in pool stats undermines trust. Tests are essential before sponsors put real funds in.

Before Starting

Read these context files first:

• context/architecture-context.md
• context/code-standards.md
• context/progress-tracker.md
• src/modules/sponsors/sponsors.service.ts

What To Build

1. Create test/unit/sponsors.service.spec.ts.
2. Test getPool(): returns aggregated { totalDeposited, totalShares, utilizationBps, apyBps }; mocks repo + contract client.
3. Test buildDepositXdr(wallet, amount): calls the liquidity client with correct args; returns the XDR; rejects amount ≤ 0.
4. Test buildWithdrawXdr(wallet, shares): rejects shares below MIN_WITHDRAWAL_SHARES; otherwise returns valid XDR.
5. Test that pool utilization is computed as (borrowed / totalDeposited) * 10_000 rounded to bps.
6. Achieve ≥ 85% coverage.

Files To Touch

• test/unit/sponsors.service.spec.ts

Acceptance Criteria

• All 3 service methods covered
• Minimum-withdrawal validation tested
• Pool math verified against a known fixture
• npm test -- sponsors.service passes
• Coverage ≥ 85%
• No any types

Mandatory Checks Before PR

• npm run build passes with zero TypeScript errors
• No new any types introduced anywhere
• Full Swagger @apioperation + @apiresponse decorators on any new endpoints
• New migration file created for any schema changes
• context/progress-tracker.md updated

[Iwayemi-Kehinde]

I would like to work on this issue as I have reviewed the scope, I should have a PR Ready in 50 minutes.

[grantfox-oss]

🦊 GrantFox — @Iwayemi-Kehinde has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #8)
2. Your PR will be reviewed by the StepFi-app maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign campaign!

@Iwayemi-Kehinde's PR #48 was approved and merged by @EmeditWeb.

🏆 @Iwayemi-Kehinde: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (48 total points). Track your full progress on GrantFox.

👏 Great work, @Iwayemi-Kehinde! Keep contributing to StepFi-app.