.NET: fix: avoid AGUI tool result message id collisions

[he-yufeng]

Summary

• generate a separate fallback message id for tool-result-only updates
• keep assistant text and tool-call parent events on the shared assistant fallback id
• add a regression test covering null provider MessageId with text, tool call, and tool result events

Fixes #5794.

To verify

• dotnet test --project .\tests\Microsoft.Agents.AI.AGUI.UnitTests -f net10.0 --filter-query "/*/Microsoft.Agents.AI.AGUI.UnitTests/AGUIStreamingMessageIdTests/ToolResults_NullMessageId_GeneratesDistinctMessageIdAsync"
• dotnet build .\src\Microsoft.Agents.AI.AGUI -f net10.0 --no-restore --tl:off
• dotnet build .\src\Microsoft.Agents.AI.Hosting.AGUI.AspNetCore -f net10.0 --no-restore --tl:off
• dotnet test --project .\tests\Microsoft.Agents.AI.AGUI.UnitTests -f net10.0 --no-restore
• dotnet run --project .\tests\Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests -f net10.0 --
• dotnet format .\src\Microsoft.Agents.AI.AGUI\Microsoft.Agents.AI.AGUI.csproj --verify-no-changes --no-restore
• dotnet format .\tests\Microsoft.Agents.AI.AGUI.UnitTests\Microsoft.Agents.AI.AGUI.UnitTests.csproj --verify-no-changes --no-restore
• dotnet format .\src\Microsoft.Agents.AI.Hosting.AGUI.AspNetCore\Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.csproj --verify-no-changes --no-restore

Note: dotnet test --project .\tests\Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests -f net10.0 --no-restore reports the repository-level MTP/VSTest runner mismatch for that project, so I ran it through dotnet run --project ... -f net10.0 -- instead.

[Copilot]

Pull request overview

This PR fixes an AG-UI streaming regression where providers that omit ChatResponseUpdate.MessageId could cause tool result events to reuse the assistant message’s fallback ID, leading to message ID collisions and front-end deduplication (Fixes #5794).

Changes:

• Adjusts AsAGUIEventStreamAsync fallback MessageId generation to avoid collisions between assistant messages and tool result messages.
• Adds ContainsOnlyToolResults helper to detect tool-result-only updates for special fallback handling.
• Adds a regression unit test covering null provider MessageId across assistant text, tool call, and tool result events.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
dotnet/src/Microsoft.Agents.AI.AGUI/Shared/ChatResponseUpdateAGUIExtensions.cs	Updates fallback message-id generation logic to avoid tool result collisions; adds helper method.
dotnet/tests/Microsoft.Agents.AI.AGUI.UnitTests/AGUIStreamingMessageIdTests.cs	Adds regression test ensuring tool results get a distinct fallback MessageId when provider omits it.

[he-yufeng]

Addressed the mixed-content tool result case in 501cae3. Fallback IDs now split whenever an update contains a FunctionResultContent, not only when it contains function results exclusively, so a tool-role update with TextContent + FunctionResultContent gets a distinct message id and the emitted tool-role text shares the tool result id.\n\nValidation:\n\npowershell\ndotnet build dotnet\\tests\\Microsoft.Agents.AI.AGUI.UnitTests\\Microsoft.Agents.AI.AGUI.UnitTests.csproj --no-restore\ndotnet dotnet\\tests\\Microsoft.Agents.AI.AGUI.UnitTests\\bin\\Debug\\net10.0\\Microsoft.Agents.AI.AGUI.UnitTests.dll --filter-method '*ToolResults_NullMessageId_GeneratesDistinctMessageIdAsync' --filter-method '*ToolResults_WithTextContent_GeneratesDistinctMessageIdAsync' --filter-method '*ToolCalls_EmptyMessageId_GeneratesFallbackParentMessageIdAsync' --no-progress\ndotnet format dotnet\\agent-framework-dotnet.slnx --verify-no-changes --no-restore --include dotnet\\src\\Microsoft.Agents.AI.AGUI\\Shared\\ChatResponseUpdateAGUIExtensions.cs dotnet\\tests\\Microsoft.Agents.AI.AGUI.UnitTests\\AGUIStreamingMessageIdTests.cs\ngit diff --check\n\n\nResult: build passed with 0 warnings/errors, 3 targeted tests passed, format and diff checks passed.

[he-yufeng]

Rebased this branch onto current main and force-pushed with lease. Re-ran the focused AG-UI validation after the rebase: AGUI unit-test project build passed with 0 warnings/errors, the three message-id regression tests passed, dotnet format --verify-no-changes passed for the touched files, and git diff --check passed.