Populate ChatMessage.MessageId in ToChatResponse and align with ResponseId

[Copilot]

AbstractionMapper.ToChatResponse returned a ChatResponse with ResponseId set, but the contained ChatMessage had no MessageId. This caused incomplete identity mapping for completed chat responses.

• Mapping fix

  • Generate the response identifier once in ToChatResponse.
  • Assign that identifier to both:
    • ChatResponse.ResponseId
    • ChatMessage.MessageId
  • Preserve existing behavior: prefer CreatedAtString, otherwise fall back to generated GUID.

• Behavioral coverage

  • Extend ToChatCompletionMethod.Maps_Known_Properties to assert that response.Messages[0].MessageId is populated from CreatedAtString.

var responseId = stream.CreatedAtString ?? Guid.NewGuid().ToString("N");
var chatMessage = ToChatMessage(stream.Message);
chatMessage.CreatedAt = stream.CreatedAt;
chatMessage.MessageId = responseId;

return new ChatResponse(chatMessage)
{
    ResponseId = responseId,
    // ...
};

Original prompt

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This section details on the original issue you should resolve

<issue_title>ChatResponseExtensions.ToChatResponse; No messageId present</issue_title>
<issue_description>

Discussed in #299

^{Originally posted by flintlouis August 14, 2025}
Hi,
When using the OllamaApiClient, after getting the IAsyncEnumerable<ChatResponseUpdate> responseStream from GetStreamingResponseAsync(), I use the ChatResponseExtensions.ToChatResponse() from Micrsoft.Extensions.AI to squash all the seperate response chunks into a ChatResponse with a list of response messages. This extension uses the MessageId to distuingish which response chunks belong to which message and seperates it in this way.
But with the ChatResponseStream there are no message ID's implemented so this extension method just adds all AIContent into 1 message, even if it has a different role.

I was wondering, is it an idea to add messageId's to the ollama Message?

Example:
Before:
responseChunks =
[
{ role: assistant, content: [text] },
{ role: assistant, content: [tool_call] },
{ role: tool, content: [tool_response] },
{ role: assistant, content: [text] }
]
After:
response.Messages =
[
{ role: assistant, content: [text, tool_call, tool_response, text] }
]

Comments on the Issue (you are @copilot in this section)

• Fixes ChatResponseExtensions.ToChatResponse; No messageId present #359

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