ToChatResponse: Merge AdditionalProperties into ChatMessage instead of ChatResponse

[Copilot]

• Understand the issue: ToChatResponse{Async} should merge ChatResponseUpdate.AdditionalProperties into the corresponding ChatMessage.AdditionalProperties rather than ChatResponse.AdditionalProperties
• Modify ProcessUpdate method in ChatResponseExtensions.cs to add AdditionalProperties based on MessageId presence
• Update existing tests in ChatResponseUpdateExtensionsTests.cs to expect the new behavior
• Add new test ToChatResponse_AdditionalPropertiesGoToMessages to verify the correct behavior
• Build and run tests to verify the changes
• Fix test failure in OpenTelemetryChatClientTests
• Add dedicated test ToChatResponse_AdditionalPropertiesRoutingBasedOnMessageId for MessageId-based routing

Summary

This PR updates ToChatResponse and ToChatResponseAsync methods to use MessageId as the discriminator for routing AdditionalProperties:

Routing logic:

• Updates with MessageId → AdditionalProperties go to message.AdditionalProperties (message-scoped)
• Updates without MessageId → AdditionalProperties go to response.AdditionalProperties (response-scoped)

Benefits:

1. Properties from distinct messages don't collide with each other when using MessageId
2. Response-level metadata (like system_fingerprint) is preserved correctly for telemetry
3. Backward compatible with existing code that doesn't use MessageId

Original prompt

See microsoft/agent-framework#3086. Please update ToChatResponse{Async} so that a ChatResponseUpdate's AdditionalProperties contribute to the ChatMessage it gets merged into rather than contributing into the top-level ChatResponse. That way, properties from distinct messages don't collide with each other and more information from the updates can be preserved.

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Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

This PR fixes a bug where AdditionalProperties from ChatResponseUpdate objects were incorrectly being merged into the top-level ChatResponse.AdditionalProperties instead of the individual ChatMessage.AdditionalProperties. This caused properties from different messages to collide when they had the same keys.

Key Changes:

• Moved AdditionalProperties merging logic from ChatResponse level to ChatMessage level in ProcessUpdate method
• Updated existing test expectations to reflect that properties now belong to messages
• Added comprehensive new test to verify properties are correctly isolated per message

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/Libraries/Microsoft.Extensions.AI.Abstractions/ChatCompletion/ChatResponseExtensions.cs	Relocated the AdditionalProperties merging logic (lines 538-548) to apply to message instead of response, enabling per-message property isolation
test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/ChatCompletion/ChatResponseUpdateExtensionsTests.cs	Updated existing test to expect AdditionalProperties on messages (lines 60-65) and added new test ToChatResponse_AdditionalPropertiesGoToMessages (lines 449-496) to verify the corrected behavior with multiple messages

[stephentoub]

@copilot, there's a test failure, e.g.

Failed	Microsoft.Extensions.AI.OpenTelemetryChatClientTests.ExpectedInformationLogged_Async(enableSensitiveData: True, streaming: True)	Microsoft.Extensions.AI.OpenTelemetryChatClientTests.Microsoft.Extensions.AI.OpenTelemetryChatClientTests.ExpectedInformationLogged_Async(enableSensitiveData: True, streaming: True)	Assert.Equal() Failure: Values differ

Expected: abcdefgh
Actual:   null
...	

[Copilot]

@copilot, there's a test failure, e.g.

Failed	Microsoft.Extensions.AI.OpenTelemetryChatClientTests.ExpectedInformationLogged_Async(enableSensitiveData: True, streaming: True)	Microsoft.Extension...

Fixed in 04ddd18. The issue was that my initial change unconditionally moved AdditionalProperties to the message. The fix now uses MessageId as the discriminator: updates with MessageId route properties to message.AdditionalProperties, while updates without MessageId (like the trailing metadata update with system_fingerprint) route to response.AdditionalProperties.

[stephentoub]

cc: @westey-m