fix: use Struct format for Gemini functionResponse

[Tarquinen]

Summary

• Fix Gemini API compatibility by using Struct (object) format for functionResponse.response instead of a plain string
• The Gemini API expects the response to be an object with name and content fields

Changes

The pruned content placeholder is now wrapped in a proper Struct object:

response: {
    name: part.functionResponse.name,
    content: PRUNED_CONTENT_MESSAGE
}

This ensures the API doesn't reject pruned tool outputs due to type mismatch.