feat: validate service import when handling backend update

[zhiying-lin]

What type of PR is this?
/kind feature

What this PR does / why we need it:
validate whether the service import when handling update backend

Which issue(s) this PR fixes:

Fixes #

Requirements:

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

How has this code been tested

integration tests

Special notes for your reviewer

updating serviceImport from invalid to valid will be tested in the future PR.

[codecov]

Codecov Report

Attention: Patch coverage is 78.37838% with 8 lines in your changes missing coverage. Please review.

Project coverage is 78.71%. Comparing base (5fb2495) to head (b9bc9a4).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...ontrollers/hub/trafficmanagerbackend/controller.go	78.37%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #210      +/-   ##
==========================================
- Coverage   78.83%   78.71%   -0.12%     
==========================================
  Files          27       27              
  Lines        3539     3576      +37     
==========================================
+ Hits         2790     2815      +25     
- Misses        600      611      +11     
- Partials      149      150       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.