.Net: Fix OpenAI connector compatibility

[JadynWong]

Motivation and Context

Fix #5337

Description

Improved openai connector compatibility.
When the user message has only one TextContent, serialize the user message as a string. Otherwise, serialize the user message as an array.

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 😄

[RogerBarreto]

@SergeyMenshykh I believe this change is not what we intent, right? Our Content should map to the first Text item of a ChatMessage,

@JadynWong, can you provide the scenario on what is the current problem and what this solves that the actual impl doesnt?

[JadynWong]

@RogerBarreto, I wrote it in #5337. The main reason for this is that we use openai connectors through third party openai compatible endpoints. They currently do not support arrays of content. PR #5088 changes prevented the use of these compatibility endpoints.

[xuzeyu91]

@SergeyMenshykh I believe this change is not what we intent, right? Our Content should map to the first Text item of a ChatMessage,

@JadynWong, can you provide the scenario on what is the current problem and what this solves that the actual impl doesnt?

Because Semantickernel currently mainly supports OpenAI and Azure OpenAI,
there are many other LLMs, and in order to enable them to also use the semantic kernel, we usually package a model connector to convert other LLMs to the same format as OpenAI,for example https://localai.io/ . However, this change may not be very friendly to other models.