Allow rename equals to target config filename on kubecm add

[poneding]

Description

I found that this is not allowed when using kubecm add to rename to the target file name.

As shown in the picture above, if I select Ture and rename it dev, it will fail.

So, this PR commited, and by the way, I fixed the bug in the previous PR that would cause the add config to fail if it had the same clustername and username as an existing context.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update (changes only affecting documentation)

Checklist

• [ x] I have tested my changes locally and ensured they are functioning properly. Please run the make build and make test commands.
• I have added/updated unit or e2e tests to cover my changes.
• I have updated the relevant documentation. If you change commands or arguments, run make doc-gen to generate new documentation.

[the-label-bot]

The Label Bot has predicted the following:

Category	Value	Confidence	Applied Label

[codecov]

Codecov Report

Attention: Patch coverage is 14.28571% with 6 lines in your changes missing coverage. Please review.

Project coverage is 19.81%. Comparing base (06a00fe) to head (b214cd3).

Files with missing lines	Patch %	Lines
cmd/add.go	14.28%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1060      +/-   ##
==========================================
+ Coverage   19.56%   19.81%   +0.24%     
==========================================
  Files          21       21              
  Lines        2647     2649       +2     
==========================================
+ Hits          518      525       +7     
+ Misses       2075     2071       -4     
+ Partials       54       53       -1     

Flag	Coverage Δ
unittests	19.81% <14.28%> (+0.24%)	⬆️

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