validator registration not working on 1.9rc3 [Teku]

on 1.9rc2

at startup teku tries to post ssz content and immediately fallback to JSON, which works

```
{"@timestamp":"2025-04-15T10:00:33,745","level":"DEBUG","thread":"OkHttp http://127.0.0.1:18550/...","class":"RestBuilderClient","message":"Failed to register validator using SSZ. Will retry using JSON (error: {"code":400,"message":"invalid character 'ò' looking for beginning of value"})","throwable":""}
{"@timestamp":"2025-04-15T10:00:33,889","level":"INFO","thread":"validator-async-2","class":"teku-validator-log","message":"Validator *** 24 out of 24 validator registration(s) were successfully sent to the builder network via the Beacon Node.","throwable":""}
```

on 1.9rc3 the endpoint keep failing with the same 502 despite the fallback is happening as before

```
{"@timestamp":"2025-04-15T09:48:59,515","level":"DEBUG","thread":"OkHttp http://127.0.0.1:18550/...","class":"RestBuilderClient","message":"Failed to register validator using SSZ. Will retry using JSON (error: {"code":502,"message":"no successful r
elay response"})","throwable":""}
{"@timestamp":"2025-04-15T09:48:59,561","level":"INFO","thread":"validator-async-3","class":"teku-validator-log","message":"Validator *** 0 out of 24 validator registration(s) were successfully sent to the builder network via the Beacon Node.","t
hrowable":""}
{"@timestamp":"2025-04-15T09:48:59,561","level":"ERROR","thread":"validator-async-3","class":"teku-validator-log","message":"Validator *** Failed to send validator registrations to the builder network via the Beacon Node","throwable":" java.util.
concurrent.CompletionException: tech.pegasys.teku.ethereum.executionclient.response.InvalidRemoteResponseException: Invalid remote response from the builder: {\"code\":502,\"message\":\"no successful relay response\"}\n\tat java.base/java.util.conc
....}
{"@timestamp":"2025-04-15T09:48:59,563","level":"INFO","thread":"validator-async-3","class":"ValidatorRegistrator","message":"Will retry to register validators in 30 seconds","throwable":""}
```