[Vertex AI] Refactor `FinishReason` as a `struct` and add new values #9084

Job	Run time
changes	4s
spm-binary-cron	0s
spm-package-resolved	1m 7s
check	0s
cmake-prod-db	0s
sanitizers-mac	0s
cmake	0s
sanitizers-ubuntu	0s
pod-lib-lint	0s
xcodebuild	0s
check-firestore-internal-public-headers	0s
spm-binary	0s
pod-lib-lint-cron	0s
spm-source	0s
Check all required Firestore tests results	0s
	1m 11s

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