feat(auth): verify API client connectivity in set_api_client

[szaher]

Add an authentication check when setting a custom API client to
catch misconfigured or invalid clients early. The verification
calls AuthenticationApi.get_api_group() to confirm the client
can reach the Kubernetes API server before proceeding.

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What changes have been made

Verification steps

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• I've made sure the tests are passing.
• Testing Strategy
  • Unit tests
  • Manual tests
  • Testing is not required for this change

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.56%. Comparing base (e5cdf08) to head (9a10717).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1067   +/-   ##
=======================================
  Coverage   96.56%   96.56%           
=======================================
  Files          23       23           
  Lines        2270     2272    +2     
=======================================
+ Hits         2192     2194    +2     
  Misses         78       78           

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[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

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