test: add unit coverage for image registry service responses

[jyun-KIM]

Proposed changes

This PR adds unit tests for the image registry service responses. The service in chaoscenter/graphql/server/pkg/image_registry/service.go exposes five methods (CreateImageRegistry, UpdateImageRegistry, DeleteImageRegistry, GetImageRegistry, ListImageRegistries) that all return model.ImageRegistryResponse, but had no unit test coverage.

The new pkg/image_registry/service_test.go wires the real Operator on top of the existing mocked MongoOperator (following the pattern in pkg/chaos_experiment/ops/service_test.go) and uses table-driven tests to assert both the mapped response fields and error propagation for every method. List is additionally covered for multi-result, empty-result, and error cases.

Local result: all subtests pass with 100% statement coverage of the package; gofmt is clean.

Fixes #5539

Types of changes

What types of changes does your code introduce to Litmus? Put an x in the boxes that apply

• New feature (non-breaking change which adds functionality)
• Bugfix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices applies)
• Test (adds unit test coverage, no production code change)

Checklist

• I have read the CONTRIBUTING doc
• I have signed the commit for DCO to be passed.
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have added necessary documentation (if appropriate)

Dependency

• None.

Special notes for your reviewer:

• No production code is modified; only a new test file is added.
• Tests can be run with: cd chaoscenter/graphql/server && go test -cover ./pkg/image_registry/...