[Cluster] Resolve ingress-nginx-controller service not found in ingress mode

[hongsu-moreh]

This PR fixes an issue where the installed ingress-nginx-controller could not be found, which prevented endpoint lookup.

• Related issue: skypilot-org/skypilot#4915

Before (output):

$ sky status --endpoints gpt-oss-20b
No endpoints exposed yet. If the cluster was recently started, please retry after a while. Additionally, make sure your Ingress is configured correctly.
To debug, run: kubectl describe ingress && kubectl describe ingressclass
Aborted!

After (output):

$ sky status --endpoints gpt-oss-20b
8000: http://192.168.2.232/skypilot/skypilot/gpt-oss-20b-cf9280d8/8000

Note:

This may still cause issues if there are multiple ingress-nginx-controller instances installed.

Tested (run the relevant ones):

• Code formatting: install pre-commit (auto-check on commit) or bash format.sh
• Any manual or new tests for this PR (please specify below)
• All smoke tests: /smoke-test (CI) or pytest tests/test_smoke.py (local)
• Relevant individual tests: /smoke-test -k test_name (CI) or pytest tests/test_smoke.py::test_name (local)
• Backward compatibility: /quicktest-core (CI) or pytest tests/smoke_tests/test_backward_compat.py (local)