feat(sponsors): add pool stats, deposit/withdraw XDR methods and unit tests

[Iwayemi-Kehinde]

Add getPool(), buildDepositXdr(), and buildWithdrawXdr() to SponsorsService

Closes #8

Overview

Replaces the existing deposit() placeholder with real contract client delegation and adds full pool aggregation and XDR builder methods with test coverage.

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Changes

src/modules/sponsors/sponsors.service.ts

• Inject LiquidityPoolContractClient into the constructor
• Add MIN_WITHDRAWAL_SHARES = 1 constant
• getPool() — aggregates totalDeposited and totalShares from the liquidity pool contract, queries active loans from Supabase, and computes:
  • utilizationBps = Math.round((totalLoaned / totalDeposited) * 10_000)
  • apyBps = Math.round(weightedAvgInterestRate * 0.85 * 100)
• buildDepositXdr(wallet, amount) — validates amount > 0, converts to stroops, delegates to liquidityClient.buildDepositTx(), returns the unsigned XDR
• buildWithdrawXdr(wallet, shares) — validates shares >= MIN_WITHDRAWAL_SHARES, converts to stroops, delegates to liquidityClient.buildWithdrawTx(), returns the unsigned XDR

src/modules/sponsors/sponsors.module.ts

• Import StellarModule so LiquidityPoolContractClient is injectable

test/unit/modules/sponsors/sponsors.service.spec.ts (new)

• getPool — verifies aggregation, utilization math, APY math, empty loan edge case, zero-pool fallback, and contract error propagation
• buildDepositXdr — verifies valid XDR, correct stroops conversion, rejection of amount ≤ 0, and contract error surfacing
• buildWithdrawXdr — verifies valid XDR, correct stroops conversion, rejection below MIN_WITHDRAWAL_SHARES, and contract error surfacing
• register / getMyPool / getStats — coverage for existing methods to reach ≥ 85%
• Zero any types used throughout

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Testing

npm test -- sponsors.service    # passes
npm run test:cov                # coverage ≥ 85%
npm run build                   # zero TypeScript errors