feat(gateway): record a client that leaves before its response exists - #54

A disconnect was recorded only after a stream had started (StreamOutcome::ClientCancelled, 499). A client that left while its key's roles and limits loaded, the pre-flight stages ran, a route was picked, or a whole (non-streamed) answer was awaited left no row at all: hyper drops the handler future and nothing after the await point runs.

How

• proxy::EarlyCancel — a drop guard. The API-key middleware arms it for the AI gateway surface as soon as the key row is known (a client that leaves before that is nobody yet and writes nothing), and disarms it after next.run returns — whatever the response, since every response and refusal path already writes its own row. The auth refusals after arming run inside the same scope, so they disarm too; only a dropped future leaves it armed.
• Dropped armed, it writes one gateway_logs row: status 499, stream_outcome: client_cancelled (the marker cancelled streams already carry), cancelled_before: response, zero tokens, zero cost, no provider.
• The handler fills in what it learns on the way through an EarlyCancelSlot request extension: the trace id and the aliased model. The fuller identity (email, IP) is filled in by the middleware once built.
• No double logging: by the time a stream exists the handler has returned and the guard is disarmed; the stream's tail records its own cancel as before. WebSocket turns pass no slot — the connection's relay records a turn the client left (as a dropped stream).

Tests (early_cancel.rs)

• Deterministic: a non-streamed request is dropped once the (60 s) upstream has received it → exactly one 499 row with the model, no tokens.
• Leaving at once (5 ms client timeout, five times) during auth/limits/routing → at least one and at most one row per client, all client_cancelled, never a success.
• A started stream the client leaves → exactly one row, without cancelled_before, and no second row later.
• The first two fail on the old code (no rows) and pass now.

Local: fmt, clippy (--all-targets, --lib), 688 unit tests, full integration suite (345 passed) on own containers.

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