Fix serialization of OpenAIPromptExecutionSettings with System.Type in ResponseFormat

[shethaadit]

Description:

This pull request addresses a serialization issue where setting the ResponseFormat property of OpenAIPromptExecutionSettings to a System.Type would cause a System.NotSupportedException during JSON serialization. This was due to System.Type not being inherently JSON serializable, which broke scenarios that rely on serializing prompt execution settings.

To resolve this, a custom JsonConverter (TypeOrFunctionResponseFormatConverter) has been introduced. This converter intercepts System.Type objects during serialization and correctly transforms them into the JSON schema format expected by the OpenAI API.

Changes:

• Created TypeOrFunctionResponseFormatConverter.cs to handle the custom serialization logic for System.Type.
• Applied the new converter to the ResponseFormat property in OpenAIPromptExecutionSettings.cs.
• Added a unit test to OpenAIPromptExecutionSettingsTests.cs to verify that serialization now works as expected without throwing an exception.

Validation:

The included unit test, ItCanSerializeResponseFormatAsType, now passes, confirming that the OpenAIPromptExecutionSettings can be successfully serialized when ResponseFormat is a System.Type.

Issue:

Fixes #13107

[markwallace-microsoft]

@shethaadit can you take a look at the unit test failure

[shethaadit]

@shethaadit can you take a look at the unit test failure

Hi @markwallace-microsoft, I fixed it and made some code change. Could you please review again?