fix(cli): use _GCLOUD_CMD for gcloud calls in GKE deploy on Windows

[anxkhn]

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Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

No existing issue; describing the change below (per CONTRIBUTING, non-doc PRs may
describe the bug directly in the PR following the issue template structure).

2. Or, if no issue exists, describe the change:

Problem:

adk deploy gke is broken on Windows. The GKE deploy path in
src/google/adk/cli/cli_deploy.py invokes gcloud via the bare name 'gcloud' in
two places:

• the gcloud builds submit call, and
• the gcloud container clusters get-credentials call.

On Windows the gcloud entrypoint is the gcloud.cmd batch script.
subprocess.run([...]) is called without a shell, and Python does not apply
PATHEXT to a bare command name in that case, so gcloud is not resolved to
gcloud.cmd. Both calls therefore raise FileNotFoundError and the deploy fails.

The module already defines, precisely for this reason:

_GCLOUD_CMD = 'gcloud.cmd' if _IS_WINDOWS else 'gcloud'

and uses it correctly in the Cloud Run deploy path. The Windows fix for Cloud Run
(#1597) introduced _GCLOUD_CMD but only updated the Cloud Run call sites; the two
GKE call sites were left as the literal 'gcloud'. This PR is the follow-up that
finishes the same fix for the GKE path.

Solution:

Point the two GKE gcloud invocations at the existing _GCLOUD_CMD constant,
mirroring the Cloud Run path. This is the smallest change that resolves the issue:
two lines. There is no behavior change on POSIX, where _GCLOUD_CMD == 'gcloud';
only Windows behavior changes (bare gcloud -> gcloud.cmd).

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Added test_to_gke_uses_gcloud_cmd_on_windows in
tests/unittests/cli/utils/test_cli_deploy.py. It mirrors the existing
test_to_gke_happy_path, patches _GCLOUD_CMD to "gcloud.cmd" to simulate
Windows, and asserts that both gcloud invocations (builds submit and
container clusters get-credentials) use gcloud.cmd as argv[0]. The test is red
on the current code (AssertionError: assert 'gcloud' == 'gcloud.cmd') and green
with the fix.

pytest summary (CLI deploy suite):

$ pytest tests/unittests/cli/utils/test_cli_deploy.py \
         tests/unittests/cli/utils/test_cli_deploy_to_cloud_run.py \
         tests/unittests/cli/utils/test_cli_deploy_ignore.py \
         tests/unittests/cli/utils/test_cli_tools_click.py -q
92 passed

Manual End-to-End (E2E) Tests:

This is a Windows-only executable-resolution fix in an external gcloud
subprocess call, so a full GKE deploy is not reproducible in CI. The failure and
fix are demonstrated deterministically by the unit test above, which asserts the
exact argv[0] passed to subprocess.run for both gcloud calls. On POSIX there is
no change, since _GCLOUD_CMD already resolves to 'gcloud'.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end. (Not feasible without a
  Windows host + a real GKE project; covered by the deterministic unit test.)
• Any dependent changes have been merged and published in downstream modules.

Additional context

This completes the Windows gcloud-invocation fix started for the Cloud Run path,
which introduced _GCLOUD_CMD but did not update the GKE call sites. After this
change, every gcloud subprocess invocation in cli_deploy.py (Cloud Run and GKE)
goes through _GCLOUD_CMD.

---

Diff summary (for reviewer reference; not part of the PR body)

• src/google/adk/cli/cli_deploy.py (+2 / -2): 'gcloud' -> _GCLOUD_CMD at the
  to_gke builds submit site and the container clusters get-credentials site.
• tests/unittests/cli/utils/test_cli_deploy.py (+51): new regression test
  test_to_gke_uses_gcloud_cmd_on_windows.

Total: 2 files, +53 / -2.