.Net: Fix Gemini Auto Invoke when returned function is non-first part (#12174)

### Motivation and Context

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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:
```json
"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

<!-- Describe your changes, the overall approach, the underlying design.
These notes will help understanding how your code works. Thanks! -->

- 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

<!-- Before submitting this PR, please make sure: -->

- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

---------

Co-authored-by: Roger Barreto <19890735+RogerBarreto@users.noreply.github.com>

Author: ionite34

dotnet/src/Connectors/Connectors.Google.UnitTests/Core/Gemini/Clients/GeminiChatStreamingFunctionCallingTests.cs

@@ -272,6 +272,35 @@ await client.StreamGenerateChatMessageAsync(chatHistory, executionSettings: exec
             Assert.False(string.IsNullOrWhiteSpace(item.Content)));
     }
 
+    [Fact]
+    public async Task IfAutoInvokeShouldReturnAssistantToolCallMessagesWithTextAsync()
+    {
+        // Arrange
+        using var handlerStub = new MultipleHttpMessageHandlerStub();
+        handlerStub.AddJsonResponse(this._responseContentWithFunction);
+        handlerStub.AddJsonResponse(this._responseContent);
+#pragma warning disable CA2000
+        var client = this.CreateChatCompletionClient(httpClient: handlerStub.CreateHttpClient());
+#pragma warning restore CA2000
+        var chatHistory = CreateSampleChatHistory();
+        var executionSettings = new GeminiPromptExecutionSettings
+        {
+            ToolCallBehavior = GeminiToolCallBehavior.AutoInvokeKernelFunctions
+        };
+
+        // Act
+        var messages =
+            await client.StreamGenerateChatMessageAsync(chatHistory, executionSettings: executionSettings, kernel: this._kernelWithFunctions)
+                .ToListAsync();
+
+        // Assert
+        var firstMessage = (GeminiStreamingChatMessageContent?)messages.FirstOrDefault();
+        Assert.NotNull(firstMessage?.ToolCalls);
+        Assert.Single(firstMessage.ToolCalls,
+            item => item.FullyQualifiedName == this._timePluginNow.FullyQualifiedName);
+        Assert.False(string.IsNullOrWhiteSpace(firstMessage.Content));
+    }
+
     [Fact]
     public async Task IfAutoInvokeShouldPassToolsToEachRequestAsync()
     {

dotnet/src/Connectors/Connectors.Google.UnitTests/TestData/chat_one_function_response.json

@@ -3,6 +3,9 @@
     {
       "content": {
         "parts": [
+          {
+            "text": "Running the TimePlugin.Now function..."
+          },
           {
             "functionCall": {
               "name": "TimePlugin%nameSeparator%Now",

dotnet/src/Connectors/Connectors.Google/Core/Gemini/Clients/GeminiChatCompletionClient.cs

@@ -197,7 +197,7 @@ public async Task<IReadOnlyList<ChatMessageContent>> GenerateChatMessageAsync(
             }
 
             state.LastMessage = chatResponses[0];
-            if (state.LastMessage.ToolCalls is null)
+            if (state.LastMessage.ToolCalls is null || state.LastMessage.ToolCalls.Count == 0)
             {
                 return chatResponses;
             }
@@ -356,12 +356,16 @@ private async IAsyncEnumerable<StreamingChatMessageContent> GetStreamingChatMess
 
                     // If function call was returned there is no more data in stream
                     state.LastMessage = messageContent;
+
+                    // Yield the message also if it contains text
+                    if (!string.IsNullOrWhiteSpace(messageContent.Content))
+                    {
+                        yield return this.GetStreamingChatContentFromChatContent(messageContent);
+                    }
+
                     yield break;
                 }
 
-                // We disable auto-invoke because the first message in the stream doesn't contain ToolCalls or auto-invoke is already false
-                state.AutoInvoke = false;
-
                 // If we don't want to attempt to invoke any functions, just return the result.
                 yield return this.GetStreamingChatContentFromChatContent(messageContent);
             }
@@ -604,11 +608,17 @@ [new GeminiChatMessageContent(role: AuthorRole.Assistant, content: string.Empty,
 
     private GeminiChatMessageContent GetChatMessageContentFromCandidate(GeminiResponse geminiResponse, GeminiResponseCandidate candidate)
     {
-        GeminiPart? part = candidate.Content?.Parts?[0];
-        GeminiPart.FunctionCallPart[]? toolCalls = part?.FunctionCall is { } function ? [function] : null;
+        // Join text parts
+        string text = string.Concat(candidate.Content?.Parts?.Select(part => part.Text) ?? []);
+
+        // Gemini sometimes returns function calls with text parts, so collect them
+        var toolCalls = candidate.Content?.Parts?
+            .Select(part => part.FunctionCall!)
+            .Where(toolCall => toolCall is not null).ToArray();
+
         return new GeminiChatMessageContent(
             role: candidate.Content?.Role ?? AuthorRole.Assistant,
-            content: part?.Text ?? string.Empty,
+            content: text,
             modelId: this._modelId,
             functionsToolCalls: toolCalls,
             metadata: GetResponseMetadata(geminiResponse, candidate));