.Net: issue-10278 : Change ChatPromptParser to enable 0-n text part instead of single value

[ThDuquennoy]

Motivation and Context

See Issue #10278

Description

ChatPromptParser.cs :
Remove method IsValidChildNodes and its call in IsValidChatMessage : A chat message is valid as long as it has a role attribute. Messages with no text child or multiple text children are now valid

ChatPromptParserTests :

• Changed 3rd invalid example since the former one is now valid
• Added tests for : Message with multiple text nodes, mixed XML content and empty XML node
  Remark : The expected behavior for mixed XML content is unclear so I kept it as it was : the content of the message node ends up in a TextContent if and only if the message has no valid text or image child node.
  So for instance, if the prompt has a message that is a mixed XML with content and a child image node, the content would be ignored and the ChatMessageContent object will have only an ImageContent item

Other remark :
ChatMessageContent.Content property only returns/sets the first TextContent item.
I thought about changing it to :

• get : return a concatenation of the TextContent items separated by \n
• set : set the first TextContent element (or add one if there is none) and remove other TextContent items

But its current behavior seems intended (it is even included in some unit tests) and I felt like such a change would have too much impact across the code. So I left it as it is. But I think that such a change could be beneficial.

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 have the same test fails with the unmodified main branch. For instance the test GettingStarted/Step1_Create_Kernel fails with ConfigurationNotFoundException : Configuration section 'OpenAI' not found. I think there are some missing config files
• I didn't break anyone 😄

[ThDuquennoy]

@microsoft-github-policy-service agree company="Chapsvision"