Fix message ordering in workflow-hosted agents

[peibekwe]

Motivation & Context

When a handoff workflow was used as an agent, non-streaming responses and saved history could move a tool result away from its matching tool call. This produced invalid conversation history, potentially causing provider errors on later turns, while streaming output appeared correct and hid the problem.

Description & Review Guide

Fixes handoff tool results being separated from their matching tool calls. Messages now preserve their produced order within each agent response while keeping concurrent responses grouped correctly.

Fixes #5720

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.

[Copilot]

Pull request overview

This PR fixes message ordering for workflow-hosted agents (especially handoff workflows) so that tool results stay adjacent to their corresponding tool calls in both non-streaming responses and persisted chat history, matching the produced order seen during streaming.

Changes:

• Update MessageMerger to preserve first-seen ordering of messages within a response and keep multi-response output grouped contiguously in first-seen response order.
• Ensure injected handoff tool results in HandoffAgentExecutor carry the originating ResponseId, so merging/history logic can keep them with the correct response.
• Add unit tests covering ordering guarantees for message IDs, identifierless segments, and handoff call/result ordering across streaming vs non-streaming and history.

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/MessageMerger.cs	Reworks merge state to keep message and response ordering stable by first observation, preventing tool call/result separation.
dotnet/src/Microsoft.Agents.AI.Workflows/Specialized/HandoffAgentExecutor.cs	Tags synthesized handoff tool-result updates with the originating response ID to preserve correct grouping/order.
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/MessageMergerTests.cs	Adds targeted unit tests for ordering invariants (first-seen order, contiguity, call/result adjacency, identifierless segments).
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/HandoffOrchestrationTests.cs	Adds end-to-end workflow-host-as-agent tests verifying call/result order for both non-streaming and streaming plus persisted history.