fix: Display the right message when adding a new variant of an existing plugin

[edgarrmondragon]

• Add failing test
• Implement fix

Related:

• Closes bug: Plugin name exists still throws "not known" #7392
• feat: when duplicate plugin found (different variant) automatically rename #7393

[netlify]

✅ Deploy Preview for meltano canceled.

Name	Link
🔨 Latest commit	96b5453
🔍 Latest deploy log	https://app.netlify.com/sites/meltano/deploys/6414ea7db1d5580008430890

[codecov]

Codecov Report

Merging #7399 (96b5453) into main (dec526b) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #7399      +/-   ##
==========================================
+ Coverage   89.11%   89.20%   +0.09%     
==========================================
  Files         294      294              
  Lines       21690    21702      +12     
  Branches     2438     2439       +1     
==========================================
+ Hits        19328    19360      +32     
+ Misses       1995     1976      -19     
+ Partials      367      366       -1     

Impacted Files	Coverage Δ
src/meltano/cli/utils.py	91.63% <100.00%> (+6.43%)	⬆️
src/meltano/core/project_plugins_service.py	91.01% <100.00%> (+0.05%)	⬆️
tests/meltano/cli/test_add.py	95.68% <100.00%> (+0.19%)	⬆️

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