Contender is failing

# Problem 1

Using `latest` causes a potentially broken draft release to be used (`v0.7.1`) and the previous two versions seem to work:
- `v0.6.0`
- `v0.7.0` (requires switching from `-l` to `--forever`)

# Problem 2

Contender needs to start making requests when the target is available (e.g. when testing builder in QEMU VM). Otherwise the whole thing fails.

# Problem 3

With `v0.7.0`, contender is complaining that some txs have failed. Maybe this is okay but it is worth an investigation. (Is 2/20 succeeded txs or failed?)
```
2026-01-12T15:43:17.694383Z WARN some spam txs failed to send: 2 / 20
2026-01-12T15:43:18.697913Z INFO preparing 20 spam payloads
2026-01-12T15:43:18.703072Z INFO prepared 20 payloads
2026-01-12T15:43:19.708988Z INFO [9] executed 20 spam tasks
2026-01-12T15:43:19.709032Z INFO all spam txs sent successfully
2026-01-12T15:43:19.776020Z INFO done. run_id: 1
2026-01-12T15:43:20.780478Z INFO preparing 20 spam payloads
2026-01-12T15:43:20.782757Z INFO prepared 20 payloads
2026-01-12T15:43:21.785788Z INFO [0] executed 20 spam tasks
2026-01-12T15:43:21.785826Z WARN some spam txs failed to send: 2 / 20
2026-01-12T15:43:22.789365Z INFO preparing 20 spam payloads
2026-01-12T15:43:22.791690Z INFO prepared 20 payloads
2026-01-12T15:43:23.796350Z INFO [1] executed 20 spam tasks
2026-01-12T15:43:23.796384Z INFO all spam txs sent successfully
2026-01-12T15:43:24.798971Z INFO preparing 20 spam payloads
2026-01-12T15:43:24.801476Z INFO prepared 20 payloads
```