fix: refactor image handling to use `@netlify/images`

[keanu-a]

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Summary

Refactors logic of the initializeImageProxy function to use @netlify/images instead of express function. Removed unit tests for the old logic as well.

Fixes #EX-536

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[github-actions[bot]]

📊 Benchmark results

Comparing with 3539b0dc68802af88a032fb3925af24174c82a09

• Dependency count: 1,074 ⬆️ 0.09% increase vs. 3539b0dc68802af88a032fb3925af24174c82a09
• Package size: 298 MB ⬆️ 0.01% increase vs. 3539b0dc68802af88a032fb3925af24174c82a09
• Number of ts-expect-error directives: 381 (no change)

[serhalp]

@copilot fix the merge conflicts and address the comment. also please let me know the extent of coverage we have in place for the image behaviour.

[Copilot]

@serhalp I've opened a new pull request, #7802, to work on those changes. Once the pull request is ready, I'll request review from you.

[serhalp]

@copilot merge main into the branch and resolve the conflicts

[Copilot]

@serhalp I've opened a new pull request, #7803, to work on those changes. Once the pull request is ready, I'll request review from you.