Update VertexAIGeminiApi generateText snippet

[google-labs-jules]

Updated VertexAiGeminiApi.kt snippet to demonstrate suspend function usage with GenerativeModel.

Changes:

• Replaced generateText(scope: CoroutineScope) with suspend fun generateText(model: GenerativeModel).
• Added necessary import for GenerativeModel.
• Removed unused imports (CoroutineScope, launch).
• Preserved snippet tags.
• Replaced placeholder [...] with // ... to ensure valid Kotlin syntax.

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PR created automatically by Jules for task 5785933852825557669 started by @kkuan2011

[google-labs-jules]

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