[dotnet-code] Clarify workflow merger metadata collection

[Michelle Clayton (michelle-clayton-work)]

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The original pull request description is below.

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Summary

Separated merged response collection from response metadata aggregation in the internal workflow message merger. This keeps the Go implementation structurally closer to the .NET MessageMerger.ComputeMerged flow, where merged responses are assembled first and response metadata is collected afterward.

.NET Reference

• dotnet/src/Microsoft.Agents.AI.Workflows/MessageMerger.cs - ComputeMerged first gathers merged responses/messages, then separately aggregates agent IDs, finish reasons, and additional properties.

Public API and Behavior

No public Go API changed. No intentional behavior change was made.

Tests

• go test ./workflow/agentworkflow

Notes

Rejected candidates: Anthropic client extension shape mostly maps to public construction APIs, so changing it risked public behavior; OpenAI FeatureIndex.cs has no close Go analogue; message unwrap/helper work was skipped because open PR #1108 already covers a [dotnet-code] message-helper area. Direct git fetch of the .NET upstream was blocked by the environment (CONNECT tunnel failed, response 403), so upstream files were inspected via the provided read-only GitHub MCP bridge instead.

Closes #1142

[Copilot]

Copilot review overview

🟢 Approval recommended

The refactor preserves existing ordering and metadata merge semantics with no public API changes.

Review effort: Balanced
Findings: None

What changed in this PR

Refactors workflow response merging to separate response assembly from metadata aggregation without changing public behavior.

Changes:

• Collects merged responses before processing metadata.
• Adds a helper for agent IDs, finish reasons, and additional properties.

File	Description
workflow/​agentworkflow/​message_merger.go	Separates merged-response collection from metadata aggregation.

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[github-actions]

API consistency review

Result	Scope
out of scope	internal-only

This PR only refactors the unexported messageMerger.ComputeMerged metadata-collection logic in workflow/agentworkflow/message_merger.go into a new unexported helper collectMergedResponseMetadata; it changes no exported Go APIs and produces identical observable output (same agentIDs, finishReasons, additionalProperties, and messages computed from the same inputs), so no cross-repo parity review is needed.

Generated by Go API Consistency Review Agent for #1177 · copilot · auto · 15.8 AIC · ⌖ 6.19 AIC · ⊞ 12K · ◷

[Quim Muntal (qmuntal)]

Don't like this ine.