test(): Added unit tests for pkg/manifest/egress.go file

[Alokzh]

Description

This PR introduces comprehensive unit tests for the egress gateway manifest install/uninstall functions in pkg/manifest

How Has This Been Tested?

The new functionality has been validated through local unit testing

go test -v ./pkg/manifest/...
go test -v ./pkg/manifest/... -cover

Tests cover:

• Resource installation and deletion
• Controller reference setup
• Custom image handling
• Duplicate operation handling

Checklist:

• The title of the PR states what changed and the related issues number (used for the release note).
• Does this PR requires documentation updates?
• I've updated documentation as required by this PR.
• I have ran go fmt
• I have updated the helm chart as required by this PR.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have tested it for all user roles.
• I have added all the required unit test cases.
• I have verified the E2E test cases with new code changes.
• I have added all the required E2E test cases.

Does this PR introduce a breaking change?

[gourishkb]

minor change required. Looks good to me otherwise

[pnavali]

@gourishkb @bharath-avesha please review

[gourishkb]

LGTM, thanks @Alokzh