Motivation and Context

MessageMerger, the internal component that folds streaming AgentResponseUpdate items into a final AgentResponse, had an implicit contract with no tests validating its ordering and grouping behavior. This created two issues:

1. Message ordering bug: When updates lacked CreatedAt timestamps, CompareByDateTimeOffset treated null timestamps as "greater than" any value, pushing untimestamped messages unpredictably to the end rather than preserving their arrival order. In multi-agent scenarios (handoff, group chat), this caused message reordering that broke conversation coherence.

2. Missing invariant documentation: The merger's guarantees were never written down, and the code contained dead state (createdTimes HashSet) suggesting abandoned functionality. Future refactors risked silently breaking the contract.

Description

This PR fixes the message ordering issue, documents the merger invariants in ADR 0026, and adds comprehensive tests to pin the expected behavior.

Bug fix in MessageMerger.CompareByDateTimeOffset:

• Changed the comparison to use insertion index as tiebreaker when timestamps are missing or equal
• When CreatedAt is null for either message, or both timestamps are equal, the comparer now falls back to the original insertion index, preserving arrival order
• Closes .NET: Workflow Host-as-Agent can improperly reorder messages in history and when running non-streaming #5720

ADR 0026 establishes three invariants:

1. Single ResponseId per turn — Hosting executors must assign a ResponseId if the agent doesn't provide one; updates with ResponseId == null are "dangling" and appended at the end
2. Output order preservation — When updates lack CreatedAt, their relative order in the merged output matches arrival order
3. Per-ResponseId grouping — Messages from each ResponseId appear as a contiguous block (no interleaving), enabling per-agent grouping in multi-agent scenarios

Cleanup:

• Removed unused createdTimes HashSet that was populated but never consumed

Test coverage added in MessageMergerTests:

• Insertion-order preservation with no timestamps
• Insertion-order preservation with mixed timestamps
• Determinism across repeated runs with mixed timestamps
• Per-ResponseId grouping for interleaved multi-agent streams
• Per-ResponseId grouping with distinct response IDs
• Function call/result ordering preservation
• FinishReason propagation

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? No — this fix preserves intended behavior while correcting a subtle ordering bug