test: add MockNetworkConditions latency/failure injection utility (closes #443)

[bilhokista]

Closes #443. Stellar Wave complexity 150 applicant: bilhokista.

Changes

• NEW tests/helpers/MockNetworkConditions.ts: applyConditions({ latencyMs, failureRate, errorType }) over vi.fn mocks or { object, method } targets, plus reset(). Fake-timers compatible (plain setTimeout promise).
• tests/payment.test.ts: slow-network payment demo block (1500ms latency succeeds; failureRate 1 rejects; reset restores).

Verification

• tsc --noEmit: clean
• vitest run tests/payment.test.ts: 59/59 pass (57 existing + 2 new)
• prettier --write applied; eslint: 0 errors (25 pre-existing as-any warnings in file, untouched)
• Isolated to tests/, zero production changes