[Vertex AI] Replace `ImagenImage` protocol with `_ImagenImage` struct #9782

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spm-binary-cron	0s
changes	4s
spm-package-resolved	1m 15s
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cmake	0s
cmake-prod-db	0s
sanitizers-mac	0s
pod-lib-lint	0s
xcodebuild	0s
sanitizers-ubuntu	0s
pod-lib-lint-cron	0s
check-firestore-internal-public-headers	0s
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spm-source	0s
Check all required Firestore tests results	0s
	1m 19s

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