.Net: issue-10278 : Change ChatPromptParser to enable 0-n text part i…

…nstead of single value (#10304)

### Motivation and Context

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See [Issue
#10278](#10278
)

### Description

<!-- Describe your changes, the overall approach, the underlying design.
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**`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

<!-- 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
- [ ] 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
- [x] I didn't break anyone 😄

---------

Co-authored-by: Thomas DUQUENNOY <[email protected]>
Co-authored-by: Mark Wallace <[email protected]>
Co-authored-by: Dmytro Struk <[email protected]>

dotnet/src/SemanticKernel.Abstractions/AI/ChatCompletion/ChatPromptParser.cs

@@ -112,19 +112,12 @@ private static ChatMessageContent ParseChatNode(PromptNode node)
     /// TagName = "message"<br/>
     /// Attributes = { "role" : "..." }<br/>
     /// optional one or more child nodes <image>...</image><br/>
-    /// content not null or single child node <text>...</text>
+    /// optional one or more child nodes <text>...</text>
     /// </remarks>
     private static bool IsValidChatMessage(PromptNode node)
     {
         return
             node.TagName.Equals(MessageTagName, StringComparison.OrdinalIgnoreCase) &&
-            node.Attributes.ContainsKey(RoleAttributeName) &&
-            IsValidChildNodes(node);
-    }
-
-    private static bool IsValidChildNodes(PromptNode node)
-    {
-        var textTagsCount = node.ChildNodes.Count(n => n.TagName.Equals(TextTagName, StringComparison.OrdinalIgnoreCase));
-        return textTagsCount == 1 || (textTagsCount == 0 && node.Content is not null);
+            node.Attributes.ContainsKey(RoleAttributeName);
     }
 }

dotnet/src/SemanticKernel.UnitTests/Prompt/ChatPromptParserTests.cs

@@ -12,7 +12,7 @@ public sealed class ChatPromptParserTests
     [Theory]
     [InlineData("This is plain prompt")]
     [InlineData("<message This is invalid chat prompt>")]
-    [InlineData("<message role='user'><text>This is invalid</text><text>chat prompt</text></message>")]
+    [InlineData("<message role='user'><text>This is an invalid chat prompt</message></text>")]
     public void ItReturnsNullChatHistoryWhenPromptIsPlainTextOrInvalid(string prompt)
     {
         // Act
@@ -148,6 +148,86 @@ public void ItReturnsChatHistoryWithValidDataImageContent()
             });
     }
 
+    [Fact]
+    public void ItReturnsChatHistoryWithMultipleTextParts()
+    {
+        // Arrange
+        string prompt = GetValidPromptWithMultipleTextParts();
+
+        // Act
+        bool result = ChatPromptParser.TryParse(prompt, out var chatHistory);
+
+        // Assert
+        Assert.True(result);
+        Assert.NotNull(chatHistory);
+
+        Assert.Collection(chatHistory,
+            c => Assert.Equal("What can I help with?", c.Content),
+            c =>
+            {
+                Assert.Equal("Hello", c.Content);
+                Assert.Collection(c.Items,
+                    o =>
+                    {
+                        Assert.IsType<TextContent>(o);
+                        Assert.Equal("Hello", ((TextContent)o).Text);
+                    }, o =>
+                    {
+                        Assert.IsType<TextContent>(o);
+                        Assert.Equal("I am user", ((TextContent)o).Text);
+                    });
+            });
+    }
+
+    [Fact]
+    public void ItReturnsChatHistoryWithMixedXmlContent()
+    {
+        // Arrange
+        string prompt = GetValidPromptWithMixedXmlContent();
+
+        // Act
+        bool result = ChatPromptParser.TryParse(prompt, out var chatHistory);
+
+        // Assert
+        Assert.True(result);
+        Assert.NotNull(chatHistory);
+
+        Assert.Collection(chatHistory,
+            c => Assert.Equal("What can I help with?", c.Content),
+            c =>
+            {
+                Assert.Equal("Hi how are you?", c.Content);
+                Assert.Single(c.Items);
+            });
+    }
+
+    [Fact]
+    public void ItReturnsChatHistoryWithEmptyContent()
+    {
+        // Arrange
+        string prompt = GetValidPromptWithEmptyContent();
+
+        // Act
+        bool result = ChatPromptParser.TryParse(prompt, out var chatHistory);
+
+        // Assert
+        Assert.True(result);
+        Assert.NotNull(chatHistory);
+
+        Assert.Collection(chatHistory,
+            c => Assert.Equal("What can I help with?", c.Content),
+            c =>
+            {
+                Assert.Null(c.Content);
+                Assert.Empty(c.Items);
+            },
+            c =>
+            {
+                Assert.Null(c.Content);
+                Assert.Empty(c.Items);
+            });
+    }
+
     [Fact]
     public void ItReturnsChatHistoryWithValidContentItemsIncludeCode()
     {
@@ -259,6 +339,50 @@ private static string GetValidPromptWithDataUriImageContent()
             """;
     }
 
+    private static string GetValidPromptWithMultipleTextParts()
+    {
+        return
+            """
+
+            <message role="assistant">What can I help with?</message>
+
+            <message role='user'>
+                <text>Hello</text>
+                <text>I am user</text>
+            </message>
+
+            """;
+    }
+
+    private static string GetValidPromptWithMixedXmlContent()
+    {
+        return
+            """
+
+            <message role="assistant">What can I help with?</message>
+
+            <message role='user'>
+                This part will be discarded upon parsing
+                <text>Hi how are you?</text>
+                This part will also be discarded upon parsing
+            </message>
+
+            """;
+    }
+
+    private static string GetValidPromptWithEmptyContent()
+    {
+        return
+            """
+
+            <message role="assistant">What can I help with?</message>
+            <message role='user'/>
+            <message role='user'>
+            </message>
+
+            """;
+    }
+
     private static string GetValidPromptWithCDataSection()
     {
         return