.Net: Fix Gemini Auto Invoke when returned function is non-first part

[ionite34]

Motivation and Context

Fixes #11651

• Currently Gemini Connectors will not Auto invoke functions when tool calls are returned in the same message with text parts, which is common with 2.5 Flash and Pro models:

"content": {
  "parts": [
    {
      "text": "Running the TimePlugin.Now function..."
    },
    {
      "functionCall": {
        "name": "TimePlugin.Now",
        "args": {
          "param1": "hello"
        }
      }
    }
  ],
  "role": "model"
}

This fixes this to now support auto kernel functions with this behavior and adds unit tests with this data.

Description

• Added unit test data for streaming responses with text & tool parts
• Changed GetChatMessageContentFromCandidate to return tools from all candidate parts instead of only the first part
• Added extra check in GenerateChatMessageAsync for empty Tool results
• Fixed stream responses in auto invoke mode with texts & tool parts not returned to the caller, added unit test

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄