fix: address a name collision risk in the MCS handling logic

[michaelawyu]

What type of PR is this?

/bug

What this PR does / why we need it:

This PR fixes a name collision situation that might lead to two MCS constantly overwriting the same derived service.

Which issue(s) this PR fixes:

N/A

Requirements:

• run make reviewable for basic local test
• Read and followed fleet-networking's Code of conduct.
• uses conventional commit messages
• [NA] includes documentation
• adds unit tests

How has this code been tested

• UTs
• ITs

Special notes for your reviewer

N/A

[codecov]

Codecov Report

❌ Patch coverage is 70.50360% with 41 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.53%. Comparing base (556170b) to head (4898673).

Files with missing lines	Patch %	Lines
pkg/controllers/multiclusterservice/controller.go	61.11%	29 Missing and 6 partials ⚠️
pkg/controllers/multiclusterservice/uniquename.go	87.75%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #397      +/-   ##
==========================================
+ Coverage   67.11%   67.53%   +0.42%     
==========================================
  Files          33       34       +1     
  Lines        3783     5012    +1229     
==========================================
+ Hits         2539     3385     +846     
- Misses       1070     1444     +374     
- Partials      174      183       +9     

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