.Net: Fix bug to allow activation of JSON mode through execution_sett…

…ings (#4774)

### Motivation and Context
Closes [#4719](#4719)

**Issue**

- When setting the `execution_settings` from YAML or `config.txt`, the
`response_format` field is not recognized, and thus the JSON mode cannot
be activated.

**Root Cause**
- The issue arises because the `ResponseFormat` property, which is of
type `object?`, stores values as `JsonElement` types when read from the
aforementioned files.
- The `CreateChatCompletionOptions` method does not account for this and
performs type checking directly against types such as `string`, leading
to improper conditional branching.
- As a result, the `ChatCompletionsOptions`'s `ResponseFormat` value
remains unassigned, preventing the activation of JSON mode.

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### Description

This PR addresses the problem by adjusting the type checks to properly
recognize the `JsonElement` objects.
This ensures that when `ResponseFormat` is set, the request sent to
OpenAI is correct, and JSON mode can be utilized.

Please let me know if any further changes or discussions are needed
regarding this pull request.
<!-- Describe your changes, the overall approach, the underlying design.
These notes will help understanding how your code works. Thanks! -->

### 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 😄

dotnet/src/Connectors/Connectors.OpenAI/AzureSdk/ClientCore.cs

@@ -785,6 +785,25 @@ private static CompletionsOptions CreateCompletionsOptions(string text, OpenAIPr
                         break;
                 }
                 break;
+
+            case JsonElement formatElement:
+                // This is a workaround for a type mismatch when deserializing a JSON into an object? type property.
+                // Handling only string formatElement.
+                if (formatElement.ValueKind == JsonValueKind.String)
+                {
+                    string formatString = formatElement.GetString() ?? "";
+                    switch (formatString)
+                    {
+                        case "json_object":
+                            options.ResponseFormat = ChatCompletionsResponseFormat.JsonObject;
+                            break;
+
+                        case "text":
+                            options.ResponseFormat = ChatCompletionsResponseFormat.Text;
+                            break;
+                    }
+                }
+                break;
         }
 
         executionSettings.ToolCallBehavior?.ConfigureOptions(kernel, options);