Docker AsyncProcess can split logs

### Description

We have observed some log lines be split into two pytest logs.

In our cpp code we have the log
```cpp
log.LogFatal() << "clock() at failed initial state transition:"
<< (static_cast(clock()) / (static_cast(CLOCKS_PER_SEC) / 1000.0)) << "ms";
```

This then can be seen on the pipeline:
https://github.com/eclipse-score/lifecycle/actions/runs/26170133165/job/76985174099?pr=194#step:5:1680
```
[2026-05-20 14:47:23.692] [INFO] [launch_manager] 2026/05/20 14:47:23.8443692 3403461 000 ECU1 NONE LM log fatal verbose 3
[2026-05-20 14:47:23.693] [INFO] [launch_manager] clock() at failed initial state transition: 38.712000 ms
```

From a first look, this might be caused by the following code:
https://github.com/eclipse-score/itf/blob/d1ff92b3930c83ddc4fd0dbbcfdc7bbce980f8af/score/itf/plugins/docker.py#L184-L188

I think the stream should be somehow buffered so that log lines are not split.

### Analysis results

_No response_

### Solution

_No response_

### Error Occurrence Rate

Sporadic

### How to reproduce

_No response_

### Supporting Information

_No response_

### Classification

Minor

### First Affected Release

not released (main)

### Last Affected Release

not released (main)

### Expected Fixed Release

before release (main)

### Category

- [ ] Safety Relevant
- [ ] Security Relevant