Panic: Unwrapping send on closed report channel causes ReceiveClosed in worker tasks under load

## Summary
When running oha with high concurrency (e.g. `-n 5000 -c 1000`) and especially with `--latency-correction` enabled, worker tasks can panic with:

```
panic at src/client.rs:1254:49: called `Result::unwrap()` on an `Err` value: ReceiveClosed
```

## Reproduction
Run:
```bash
oha 'http://localhost:9000/api/requests/' -n 5000 -c 1000 -m POST --latency-correction -H 'Content-Type: application/json' -d '{"email":"test+999@mail.com"}'
```
The panic is much more likely at high concurrency or when early shutdown occurs. It is triggered when the result receiver closes before all worker tasks finish.

## Root cause
Code at [src/client.rs:1245-1250](https://github.com/hatoo/oha/blob/master/src/client.rs):
```rust
while counter.fetch_add(1, Ordering::Relaxed) < n_tasks {
let res = client.work_http1(&mut client_state).await;
let is_cancel = is_cancel_error(&res);
report_tx.send(res).unwrap();
if is_cancel { break; }
}
```
When the receiver is closed (for example, when test duration or parent task completes), further sends panic workers via `.unwrap()`.

## Impact
This is a race between result reporting and the receiver's lifetime. In normal or high-pressure use, fast termination of the receiver (parent) leaves workers panicking on `.unwrap()` if they race to send results after closed.

## Possible solutions
- Change all `report_tx.send(...).unwrap();` sites to handle channel closure gracefully, e.g.:
```rust
if report_tx.send(res).is_err() { break; }
```
- Prefer `ok()` and silent exit when the receiver is gone, since there's nothing further the worker can do.
- Audit all other worker/reporting codepaths for `send(...).unwrap()` (other than in tests) and apply similar fix.

Let me know if you need details or logs would be useful.