fix(maintenance-pool): restore balance on poolWithdraw failure

[Fabulouz34]

Summary

MaintenancePoolService.assignReward() atomically decremented pool.balance before calling
escrowService.poolWithdraw(). Any throw inside poolWithdraw — escrow not LOCKED, invalid
amount, recipient/user mismatch, Soroban simulate/send/poll failure, or a paymentRepo.save
error — left the balance permanently reduced even though no USDC ever left the contract. Over
time this caused the DB balance to drift below the real on-chain balance, eventually blocking
legitimate rewards with "exceeds pool balance" with no admin path to reconcile.

Changes

EscrowService (escrow.service.ts)

• Added public assertPoolWithdrawPreconditions() that exposes the three cheap guards already
  inside poolWithdraw (assertLocked, assertValidAmount, assertRecipientsMatchUsers) so callers
  can invoke them before touching any state.
• Fixed a pre-existing syntax error in invokeRelease() (duplicate return statement from an
  earlier partial edit that broke compilation).

MaintenancePoolService.assignReward() (maintenance-pool.service.ts)

• Calls assertPoolWithdrawPreconditions() before the atomic UPDATE … SET balance = balance -
  :amount query. Bad input (malformed amount, escrow not LOCKED, recipient/user mismatch) is now
  rejected without ever touching the DB balance.
• Wraps the poolWithdraw() call in try/catch: on any throw, `poolRepo.increment({ id },
  'balance', Number(amount)) atomically restores the reservation before rethrowing the original
  error. The escrow stays LOCKED in all failure paths — no USDC moved — so the increment
  faithfully returns the pool to its real on-chain state.

Tests (maintenance-pool.service.spec.ts)

Added a describe('balance restoration on poolWithdraw failure') block with 6 regression tests:

• poolWithdraw throwing a Soroban error → increment is called with the correct amount
• poolWithdraw throwing a DB save error → increment is called
• The exact decremented amount (including decimals) is always restored
• A successful poolWithdraw does not trigger increment
• The original error object is rethrown unchanged (not wrapped)
• A pre-decrement precondition failure neither decrements nor triggers a restore

All 22 unit tests pass.

Testing

npx jest src/maintenance-pool/maintenance-pool.service.spec.ts

22 passed, 0 failed

Closes #459

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