atelet: Bound background snapshot uploads with a timeout - #2831

… request times out (agent-substrate#2940)

Fixes agent-substrate#2910.

`DetachActor` ran its detach on the RPC's context. When a resume moved
an actor off a full node (agent-substrate#2881) and the caller left mid-upload, for
example when the router's resume budget ran out, the upload was
cancelled and the next resume started it over. A snapshot slower to
upload than that budget never moved. And because `DetachActor` cancelled
atelet's report, nothing corrected the record if the caller failed to
write it.

Now the detach runs on the tracker's context and always makes atelet's
report due.

```mermaid
sequenceDiagram
    participant A as ate-api-server
    participant L as atelet
    participant O as object storage
    A->>L: DetachActor
    L->>O: upload snapshot
    Note over A: resume budget runs out
    L-->>A: Canceled
    Note over L: the detach keeps running
    O-->>L: upload done
    L->>L: delete local copies
    L->>A: ReportActorSnapshotState
    A->>A: record the detach
```

- If the detach finishes in time, `DetachActor` returns its state as
before. Otherwise it returns the caller's error and the detach runs on.
A second call waits for it.
- Every detach makes the report due, including when the upload succeeds
and the prune fails.
- The upload timeout from agent-substrate#2831 now bounds a `DetachActor` call's
upload, since the caller's context no longer does.
- atelet's shutdown waits for a detach that outlived its call. Terminate
cancels it and waits, as agent-substrate#2943 does for any detach.

#### API changes

- `Atelet.DetachActor` (atelet internal). The proto comment now says
atelet reports the detach and that the detach outlives the call. Comment
only, with the regenerated `atelet_grpc.pb.go`.

#### Worth knowing

- Each move now sends one redundant report. It gets `Aborted` while the
actor is RESUMING, then `NotFound` once it runs elsewhere, and atelet
drops it.
- agent-substrate#2831's subtest asserted that a `DetachActor` call ignores the upload
timeout. It now asserts the timeout applies.