vertex-ai/gemini Combine messages with the same role in succession to handle multi turn messages

[narengogi]

Title:

• Gemini on Vertex AI throws an exception, when there is an message part with empty string
• Also updated tool_config keys to snake_case (it works with camelCase also, but official documentation is in snake_case)

{
    "error": {
        "message": "vertex-ai error: Unable to submit request because it has an empty text parameter. Add a value to the parameter and try again. Learn more: https://cloud.google.com/vertex-ai/generative-ai/docs/model-reference/gemini",
        "type": "INVALID_ARGUMENT",
        "param": null,
        "code": "400"
    },
    "provider": "vertex-ai"
}

• Gateway appends a message with empty string part in between two messages of the same role and it's failing thus
• Handling this by merging the messages with the same role, as gemini supports multiple parts inside the same message

{
  "model": "gemini-1.5-flash-001",
  "contents": [
    {
      "role": "user",
      "parts": [
        {
          "text": "<|context_start|>\n(Context omitted for historical message)\n<|context_end|>\n\nhello what are the discounts?"
        },
        {
          "text": "I am hungry"
        }
      ]
    },
    {
      "role": "model",
      "parts": [
        {
          "text": "That's okay, all of us get hungry sometimes?"
        }
      ]
    }
  ],
  "systemInstruction": {
    "parts": [
      {
        "text": "You're Sidekick"
      }
    ],
    "role": "system"
  },
  "generationConfig": {
    "temperature": 0.1
  }
}

Related Issues: (optional)

• Gemini on Vertex AI throws an exception, when there is an message part with empty string #462

Testing Done:

• Tested with user and model messages in succession
• Tested with assistant messages with tool responses and tool messages, verified correct exception is thrown