Expand pay() test coverage: timeout, 402 retry, insufficient balance

[RBKunnela]

Follow-up from #8 review. CodeRabbit's last nitpick on PR #8 flagged that tests/client.test.ts has strong happy-path and generic-failure coverage for pay(), but three payment-flow edges are still untested:

1. Timeout — mockFetch rejects with new Error('ETIMEDOUT') on both attempts; assert client.pay returns success: false and error contains ETIMEDOUT.
2. 402 retry — mockFetch first returns 402 { error: 'Payment required', code: 'PAYMENT_REQUIRED' }, then succeeds on retry; assert success: true and mockFetch was called twice.
3. Insufficient balance — verify returns valid with settlementToken, but settle returns 402 { error: 'Insufficient balance', code: 'INSUFFICIENT_FUNDS' }; assert success: false and errorCode === 'INSUFFICIENT_FUNDS'.

All three should follow the same mockFetch / client.pay shape as the surrounding tests, with body inspection via mockFetch.mock.calls.

Good first issue for a TS contributor. Not a blocker for #8 merge — kept that PR receipt-scoped.

Per coding guidelines tests/**: covers "Coverage of payment flow edge cases (402 retry, balance insufficient, timeout)".