feat(server): add getBalance() for balance lookups

[emilioacc]

Summary

Add getBalance() to @atxp/server so MCP servers can check a user's available balance without charging. This calls the auth server's new POST /balance endpoint (merged in auth#227).

Usage

import { getBalance, requirePayment } from '@atxp/server';

// Pre-flight: check if user can afford the upper bound
const balance = await getBalance();
if (balance.lt(upperBoundCost)) {
  // Trigger 402 payment required flow
}

// Do expensive work...
const result = await generateImage(prompt);

// Charge actual cost
await requirePayment({ price: actualCost });

Changes

packages/atxp-server

• src/types.ts — Add BalanceRequest type, add getBalance to PaymentServer interface
• src/paymentServer.ts — Implement getBalance in ATXPPaymentServer (calls POST /balance)
• src/getBalance.ts — New exported helper that uses ATXP context (same pattern as requirePayment)
• src/index.ts — Export getBalance and BalanceRequest
• src/serverTestHelpers.ts — Update mock paymentServer() to include getBalance
• Bump version to 0.10.7

Tests

• paymentServer.test.ts — Tests for ATXPPaymentServer.getBalance (success, error, missing field)
• getBalance.test.ts — Tests for the exported helper (context handling, token passing, error propagation)

Part of

ATXP-1712 — Part 2 of 3 (auth server ✅ → SDK → image service)