feat(flashtestations):  more resilient registration

when multiple builders start at the same time they end up in a race condition over the funding key's nonce:

```
server returned an error response: error code -32000: nonce too low: next nonce 14, tx nonce 13
```

`op-rbuilder` should recognise recoverable errors (like "nonce too low" or "already known") and keep retrying with capped exponential backoff.