.NET: fix: avoid mutating handoff message roles

[he-yufeng]

Summary

• stop handoff role reassignment from mutating shared conversation messages in place
• keep the existing target-agent input behavior: other agents' text context is still presented as user context
• add unit and handoff regression coverage for the non-mutating copy path

Notes

This addresses the in-place role mutation root cause called out in #5804. The target agent still receives the same role-adjusted context, but the shared conversation keeps prior agent responses as assistant messages during and after invocation.

To verify

• dotnet test --project .\tests\Microsoft.Agents.AI.Workflows.UnitTests\Microsoft.Agents.AI.Workflows.UnitTests.csproj -f net10.0 --no-build -- --filter-class Microsoft.Agents.AI.Workflows.UnitTests.AIAgentsAbstractionsExtensionsTests
• dotnet test --project .\tests\Microsoft.Agents.AI.Workflows.UnitTests\Microsoft.Agents.AI.Workflows.UnitTests.csproj -f net10.0 --no-build -- --filter-class Microsoft.Agents.AI.Workflows.UnitTests.HandoffOrchestrationTests
• dotnet format .\tests\Microsoft.Agents.AI.Workflows.UnitTests\Microsoft.Agents.AI.Workflows.UnitTests.csproj --include .\src\Microsoft.Agents.AI.Workflows\AIAgentsAbstractionsExtensions.cs .\src\Microsoft.Agents.AI.Workflows\Specialized\HandoffAgentExecutor.cs .\tests\Microsoft.Agents.AI.Workflows.UnitTests\AIAgentsAbstractionsExtensionsTests.cs .\tests\Microsoft.Agents.AI.Workflows.UnitTests\HandoffOrchestrationTests.cs --verify-no-changes --no-restore

[Copilot]

Pull request overview

This PR fixes a .NET handoff orchestration bug where role reassignment (assistant→user for “other agents” context) was mutating shared conversation message instances in-place, causing downstream consumers (e.g., telemetry) to observe agent responses incorrectly labeled as User. The change preserves the existing “handoff target sees other agents as user context” behavior while ensuring the shared conversation history remains unchanged.

Changes:

• Replace in-place role reassignment with a non-mutating copy path for messages sent to the invoked agent during handoff.
• Remove the prior “mutate + reset” mechanism in favor of a single copy-and-adjust step.
• Add unit/regression tests to ensure role reassignment does not mutate shared conversation messages and that the handoff target receives the adjusted role context.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
dotnet/src/Microsoft.Agents.AI.Workflows/Specialized/HandoffAgentExecutor.cs	Switches to a non-mutating message copy for role-adjusted agent input and removes the reset step.
dotnet/src/Microsoft.Agents.AI.Workflows/AIAgentsAbstractionsExtensions.cs	Introduces CopyWithAssistantToUserForOtherParticipants and removes the previous in-place change/reset helpers.
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/HandoffOrchestrationTests.cs	Adds a regression test verifying shared conversation messages are not mutated during handoff role reassignment.
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/AIAgentsAbstractionsExtensionsTests.cs	Adds a focused unit test ensuring the copy path does not mutate the original message and produces a distinct instance with the adjusted role.