Extracting bad log_excerpt "': File exists\n"

e.g. https://d.kernelci.org/issue/maestro%3A5b8b0e14fb5c4341239b6e160d2f4453d8c11905

```
04:41:39 CC acct_syscall
04:41:39 CC tags_test
04:41:43 cp: cannot create regular file '/tmp/kci/artifacts/build/kselftest/arm64/signal/fake_sigreturn_bad_magic': File exists
04:41:43 make[2]: *** [Makefile:21: /tmp/kci/artifacts/build/kselftest/arm64/signal/fake_sigreturn_bad_magic] Error 1
04:41:43 make[2]: Target 'all' not remade because of errors.
```

becomes

```
{
"error": {
"script": "Makefile:21",
"target": "/tmp/kci/artifacts/build/kselftest/arm64/signal/fake_sigreturn_bad_magic",
"error_type": "kbuild.other",
"log_excerpt": "': File exists\n",
"signature_fields": {
"script": "Makefile:21",
"target": "/tmp/kci/artifacts/build/kselftest/arm64/signal/fake_sigreturn_bad_magic",
"error_type": "kbuild.other",
"error_summary": ""
}
},
"parser": "kbuild",
"version": "1.0.0"
}
```

Start with the kbuild parser and reproduce the example from the linked KernelCI issue. Trace how the cp error is extracted into log_excerpt, then verify the result preserves the relevant error text and matches the expected JSON structure shown here.

Written by the indexing model from the issue text.