ECS exec: long output has linebreaks in the middle of a line

When running a command with a lot of output (which exceeds the os buffer), buffered output is not stitched together correctly. We naively append a newline after each stream batch.

For example in the following result of `find`, the line `./sys/module/ghash_clmulni_intel/sections/.rodata.str1.1` is broken into two lines, but it should be a single line.

```
...
./sys/module/ghash_clmulni_intel/sections/.text
./sys/module/ghash_clmulni_intel/sections/.rodata.cst16.bswap_mask
./sys/module/ghash_clmulni_intel/sections/.data
./sys/module/ghash_clmulni_intel/sectio
ns/.rodata.str1.1
./sys/module/nvme_core
./sys/module/nvme_core/uevent
```

Locate the ECS exec output buffering and stream-batch handling, then reproduce the issue with a command such as find whose output exceeds the OS buffer. Verify that batches are stitched together without inserting linebreaks inside a line, while genuine linebreaks remain intact.

Written by the indexing model from the issue text.