test(redis-worker): drop vi.fn handler spies from drainer tests

Replace each vi.fn(async handler) with a plain async closure that
records calls via captured counter/array variables. Assertions move
from handler.mock.* / toHaveBeenCalled* matchers to checks against
the captured state, e.g. handlerCalls.length / handlerCalls[0].
Functionally equivalent; aligns with the package convention of using
real testcontainers + closure-based probes (cf. mollifierGate.test.ts
and mollifierTripEvaluator.test.ts) rather than vitest fakes.

Author: d-cs