Fix issue where response with text and tool_calls does not return tool calls

[bennapp]

tool_calls would return nil for gemini if the response contained parts that had a text response as the first part but then subsequent tool_calls. Now, we fix that by checking if any of the parts of a functionResponse.

This handles a response like this:

{
  "candidates": [
    {
      "content": {
        "parts": [
          {"text": "Let's add those numbers"},
          {
            "functionCall": {
              "name": "calculator__execute",
              "args": {"input": "2+2"}
            }
          }
        ],
        "role": "model"
      },
      "finishReason": "STOP",
      "index": 0,
      "safetyRatings": []
    }
  ]
}