feat: support weight 0 for the trafficManagerBackend

[zhiying-lin]

What type of PR is this?

/kind feature

What this PR does / why we need it:
To support disabling the endpoint without deleting the trafficManagerBackend use case

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

Special notes for your reviewer

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 79.80%. Comparing base (929b956) to head (89c63ff).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...ontrollers/hub/trafficmanagerbackend/controller.go	62.50%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #246      +/-   ##
==========================================
- Coverage   79.92%   79.80%   -0.12%     
==========================================
  Files          27       28       +1     
  Lines        3851     3863      +12     
==========================================
+ Hits         3078     3083       +5     
- Misses        616      623       +7     
  Partials      157      157              

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[Copilot]

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Comments suppressed due to low confidence (2)

pkg/common/defaulter/trafficmanagerbackend_test.go:57

• Add a test case to ensure that SetDefaultsTrafficManagerBackend handles the scenario where the weight is explicitly set to 0.

}

pkg/controllers/hub/trafficmanagerbackend/controller.go:259

• [nitpick] The variable name 'desiredEndpointsMaps' is redundant. Consider renaming it to 'desiredEndpoints'.

desiredEndpointsMaps, invalidServicesMaps, err := r.validateExportedServiceForServiceImport(ctx, backend, serviceImport)