chore: respect namespace in report cluster

[shanshanying]

Summary

• Backport the report cluster namespace fix from PR fix: respect namespace set by user #613 to release-1.0.
• Preserve the namespace passed by -n/--namespace instead of overwriting it with the current kubeconfig namespace.
• Add a regression test for the explicit namespace case.

Validation

• GOCACHE=/private/tmp/kbcli-go-test-cache-release10 go test ./pkg/cmd/report -count=1

[codecov-commenter]

Codecov Report

❌ Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 39.93%. Comparing base (b49962c) to head (9276b63).

Files with missing lines	Patch %	Lines
pkg/cmd/report/report.go	40.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           release-1.0     #691      +/-   ##
===============================================
- Coverage        39.94%   39.93%   -0.01%     
===============================================
  Files              172      172              
  Lines            24026    24027       +1     
===============================================
- Hits              9597     9595       -2     
- Misses           13308    13310       +2     
- Partials          1121     1122       +1     

Flag	Coverage Δ
unittests	39.93% <40.00%> (-0.01%)	⬇️

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