As a developer, I want comprehensive unit tests for the HAProxy image override mechanism, so that I can ensure the feature works correctly.

Context:
Add comprehensive unit tests to verify the environment variable override mechanism works correctly for both the shared ingress router and worker node HAProxy configurations.

Tasks:

• Add tests in support/images/envvars_test.go (create if needed)
• Test default behavior when env var is not set
• Test override behavior when env var is set
• Test shared ingress router deployment with and without override
• Test worker node HAProxy configuration with and without override
• Test worker node HAProxy with env override and different Nodepool haproxy image setting

Acceptance Criteria:

• Tests cover both set and unset env var scenarios
• Tests verify correct image is used in shared ingress router
• Tests verify correct image is used in worker node HAProxy
• All tests pass