System instruction does not work when creating a context cache.

[Crymzix]

Description of the bug:

Declaring a system instruction results in an error when trying to create a context cache:

const cache = await this.genAICache.create({
      model: "models/gemini-1.5-pro-001",
      systemInstruction: "You are an expert software engineer.",
      contents: [
          {
              role: 'user',
              parts,
          },
      ],
})

Actual vs expected behavior:

Actual: GoogleGenerativeAIFetchError: [GoogleGenerativeAI Error]: Error fetching from https://generativelanguage.googleapis.com/v1beta/cachedContents: [400 Bad Request] Invalid value at 'cached_content.system_instruction' (type.googleapis.com/google.ai.generativelanguage.v1beta.Content), "You are an expert software engineer." [{"@type":"type.googleapis.com/google.rpc.BadRequest","fieldViolations":[{"field":"cached_content.system_instruction","description":"Invalid value at 'cached_content.system_instruction' (type.googleapis.com/google.ai.generativelanguage.v1beta.Content), "You are an expert software engineer.""}]}]

Expected: Cache should be created successfully.

Any other information you'd like to share?

No response