Support text response in predictions response modifier

What does this MR do and why?

When Gitlab::Llm::VertexAi::Client#text is used and the response is passed to Gitlab::Llm::VertexAi::ResponseModifiers::Predictions, #response_body will return nil. This is because it doesn't support the response format when using #text.

Example response when using #text:

{
  "predictions": [
    {
      "citationMetadata": { "citations" : [] },
      "safetyAttributes": {
        "categories": [],
        "blocked": false,
        "scores": []
      },
      "content": "content"
    }
  ]
}

This modifies the predictions response modifier to support response format.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #409509 (comment 1433293217)