Prevent additional newline in job in attach mode

What does this MR do?

This MR makes sure that no additional newlines are added in the job log with executorkubernetes in attach mode.

Helper adds a newline when the reader buffer doesn't end with one. Those additional newlines are not removed when streaming the log to the GitLab UI impacting the rendering of the log.

This MR makes sure that those newline are removed before the log is sent to the GitLab UI.

Why was this MR needed?

To make sure the job log is accurate and doesn't contain additional newline.

What's the best way to test this MR?

gitlab-ci

variables:
  FF_USE_POWERSHELL_PATH_RESOLVER: "true"
  FF_RETRIEVE_POD_WARNING_EVENTS: "true"
  FF_PRINT_POD_EVENTS: "true"
  FF_SCRIPT_SECTIONS: "true"
  CI_DEBUG_SERVICES: "true"
  GIT_DEPTH: 5
  MY_TEST_VARIABLE_1: gitlab-ci
  MY_TEST_VARIABLE_2: gitlab-ci
  SAST_GOSEC_LEVEL: 2
  END_LOOP: 50

test:
  stage: test
  
  script:
    - |
      for i in $(seq 1 120); do
        printf .
        sleep 0.02
      done

config.toml

concurrent = 1
check_interval = 1
log_level = "debug"
shutdown_timeout = 0

listen_address = ':9252'

[session_server]
  session_timeout = 1800

[[runners]]
  name = "investigation"
  url = "https://gitlab.com/"
  id = 0
  token = "glrt-REDACTED"
  token_obtained_at = "0001-01-01T00:00:00Z"
  token_expires_at = "0001-01-01T00:00:00Z"
  executor = "kubernetes"
  shell = "bash"
  limit = 1
  builds_dir = "/my_custom_dir"
  [runners.kubernetes]
    host = ""
    bearer_token_overwrite_allowed = false
    image = "alpine"
    pod_termination_grace_period_seconds = 0
    namespace = ""
    namespace_overwrite_allowed = ""
    pod_labels_overwrite_allowed = ""
    service_account_overwrite_allowed = ""
    pod_annotations_overwrite_allowed = ""
    node_selector_overwrite_allowed = ".*"
    allow_privilege_escalation = false
    [[runners.kubernetes.volumes.empty_dir]]
          name = "repo"
          mount_path = "/my_custom_dir"
    [runners.kubernetes.build_container_security_context]
      run_as_user = 1000
      run_as_group = 65533
    [[runners.kubernetes.services]]
    [runners.kubernetes.dns_config]
    [runners.kubernetes.pod_labels]
      user = "ratchade"

Job passes and no additional newline in the log

Running with gitlab-runner development version (HEAD)
  on investigation REDACTED, system ID: s_REDACTED
  feature flags: FF_USE_POWERSHELL_PATH_RESOLVER:true, FF_SCRIPT_SECTIONS:true, FF_PRINT_POD_EVENTS:true
Preparing the "kubernetes" executor
00:00
WARNING: Namespace is empty, therefore assuming 'default'.
Using Kubernetes namespace: default
Using Kubernetes executor with image alpine ...
Using attach strategy to execute scripts...
Preparing environment
00:04
Using FF_USE_POD_ACTIVE_DEADLINE_SECONDS, the Pod activeDeadlineSeconds will be set to the job timeout: 30m0s...
Subscribing to Kubernetes Pod events...
Type     Reason      Message
Normal   Scheduled   Successfully assigned default/runner-REDACTED-project-25452826-concurrent-0-x3nczs0a to minikube
Normal   Pulled   Container image "registry.gitlab.com/gitlab-org/gitlab-runner/gitlab-runner-helper:x86_64-latest" already present on machine
Normal   Created   Created container init-permissions
Normal   Started   Started container init-permissions
Normal   Pulling   Pulling image "alpine"
Normal   Pulled   Successfully pulled image "alpine" in 324ms (324ms including waiting). Image size: 4090801 bytes.
Normal   Created   Created container build
Normal   Started   Started container build
Normal   Pulled   Container image "registry.gitlab.com/gitlab-org/gitlab-runner/gitlab-runner-helper:x86_64-latest" already present on machine
Normal   Created   Created container helper
Normal   Started   Started container helper
Running on runner-REDACTED-project-25452826-concurrent-0-x3nczs0a via ratchade-MBP...
Getting source from Git repository
00:02
Fetching changes with git depth set to 5...
Initialized empty Git repository in /my_custom_dir/ra-group2/playground-bis/.git/
Created fresh repository.
Checking out f5d9fae3 as detached HEAD (ref is master)...
Skipping Git submodules setup
Executing "step_script" stage of the job script
00:03
$ for i in $(seq 1 120); do
  printf .
  sleep 0.02
done
........................................................................................................................
Cleaning up project directory and file based variables
00:00
Job succeeded

What are the relevant issue numbers?

close #27099 (closed)