fix: Add Context Cache support for ChatSessionPreview class

[oleksii-oleniuk-star]

Related issue

#395

How to reproduce a problem:

const vertexAi = new VertexAI({ project: projectId, location });
const options: ModelParams = {
    model,
    generationConfig,
    safetySettings,
};
const generativeModel = await vertexAi.preview.getGenerativeModelFromCachedContent(
    {
        name,
        model: "projects/${projectId}/locations/${location}/publishers/google/models/${modelId}",
    },
    options,
);

const chat = generativeModel.startChat({ history }); // here we receive an instance of ChatSessionPreview class

const streamGeneratedContentResult = await chat.sendMessageStream(lastMessageContent);
return streamGeneratedContentResult.stream;

Before this PR

Gemini Context Cache was ignored by ChatSessionPreview

After this PR

Gemini Context Cache works properly with ChatSessionPreview

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