prisma-risk / tsoracle / 26415651253

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feat(client): count waiters abandoned before delivery (#437)

The driver delivered every waiter's result with `let _ = respond.send(...)`.
The send fails only when the caller dropped its `Driver::request` future
(a client-side timeout or cancellation) before the result arrived, so
swallowing the outcome is correct — but it was unmeasured, leaving a
cancellation storm completely invisible.

Route all four delivery sites (success, protocol-violation fanout, error
fanout, and the synchronous InvalidCount rejection) through a single
`reply_to_waiter` helper that increments a new
`tsoracle.client.driver.abandoned_waiters.total` counter whenever the send
fails. Centralizing delivery also makes it structurally impossible to add a
new send site that forgets the metric.

The counter helper mirrors the existing driver gauges: feature-gated behind
`metrics`, a no-op otherwise, and cheap on the hot path because the counter
is only touched on the cold cancellation branch.

Add deterministic unit tests that drive the synchronous helper directly
against a dropped vs. live receiver (the spawned `driver_task` can't be
observed by the thread-local test recorder), document the signal in
operations.md, and note in the end-to-end metrics catalog why this one signal
is covered by unit tests rather than the timing-dependent end-to-end harness.

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