[CI] Migrate L4 jobs to l4-k8s and split by GPU count (#6890)

Signed-off-by: wangyu <410167048@qq.com>
Co-authored-by: amy-why-3459 <wuhaiyan17@huawei.com>

docs/contributing/ci/ci_settings.md

@@ -414,10 +414,12 @@ settings in `pyproject.toml`: online tests launch the server with
 `subprocess.Popen` and stop it with SIGTERM, and without those settings the XML
 reflects only the pytest parent process, not the server's code paths.
 
-The upload depends on `buildkite-agent` being callable inside the container. The
-`kubernetes` presets (`h100_*`, `*_npu_*`) provide it; the `docker` ones (`l4_*`)
-only do because they set `mount-buildkite-agent: true`. A new docker preset that
-runs a coverage job needs the same.
+The upload depends on `buildkite-agent` being callable inside the container.
+CUDA (`l4_*`, `h100_*`) and NPU (`*_npu_*`) presets all use the `kubernetes`
+plugin, which provides the agent. `l4_*` jobs run on the EKS `l4-k8s` queue
+(agent-stack-k8s); they no longer use the docker plugin or
+`mount-buildkite-agent`. A new **docker** preset that runs a coverage job
+still needs `mount-buildkite-agent: true`.
 
 List both `run_cov_split.sh` and `pyproject.toml` in every opted-in job's
 `source_file_dependencies` — both change what the job measures, so without them a