fix(drivers): mock's failWhen predicate always receives index 0 on send() #113

What happens

mock({ failWhen }) is documented as "fail only the messages this predicate matches — for testing partial batch failures". In the send() path the index argument is hardcoded to 0, so an index-based predicate behaves differently depending on how the message was sent.

Reproduction

const seen: number[] = []
const driver = mock({ failWhen: (_m, i) => { seen.push(i); return false } })
const email = createEmail({ driver, defaults })
await email.send({ ...msg, subject: "1" })
await email.send({ ...msg, subject: "2" })
await email.send({ ...msg, subject: "3" })

Observed:

indices over 3 sends: [0,0,0]

So mock({ failWhen: (m, i) => i === 1 }) fails the second message of a sendBatch and never fires for send().

Why it matters

This is the driver people reach for when writing tests, and a test helper that quietly does nothing is worse than one that is missing — the test passes and proves nothing. The asymmetry is invisible: the same predicate behaves differently depending on the batch size the core happened to choose.

Where

src/drivers/mock.ts:76 — opts.failWhen?.(msg, 0) in send, versus the real index in sendBatch at line 89.

Suggested fix

Either give send() a per-instance running counter so the index keeps meaning "which message", or drop the index parameter and let predicates match on the message. Matching on the message is the honest option, since a single send() has no meaningful position.