This pull request introduces improvements to how message IDs are assigned and handled for streaming agent responses, ensuring that all related updates share a stable identifier and that completion events do not duplicate content. It also adds and updates tests to verify the correct assignment and propagation of message and response IDs, and corrects the emission of completion events in streaming workflows.

Stable Message ID Assignment and Propagation:

• Streaming agent response updates now receive a stable MessageId that is reused for all updates belonging to the same logical message, ensuring consistent identification across updates. This applies both in AgentProviderExtensions.InvokeAgentAsync and AIAgentHostExecutor. [1] [2]

• The SendActivityExecutor now assigns both MessageId and ResponseId to messages and updates, and ensures these are correctly propagated through all related events.

Workflow Session and Completion Event Handling:

• The workflow session tracks streamed message IDs and, when emitting completion events, avoids duplicating content that has already been streamed. Instead, it emits a completion event with empty contents for observability. [1] [2] [3]

Test Enhancements and Coverage:

• Unit tests have been updated and expanded to verify that all streaming updates for a message share the same MessageId, and that response events reference the correct IDs. New tests also ensure that auto-send behavior is correctly honored and that completion events are emitted as expected without content duplication. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

Test Infrastructure and Mocking:

• Test agent and provider implementations have been adjusted to better simulate missing MessageId scenarios and to validate the new stable message ID logic.

Minor Code Cleanups:

• Added missing using System; directives in relevant files. [1] [2]

These changes collectively improve the reliability and clarity of agent response streaming, making it easier to track and correlate updates and final responses throughout the workflow system.

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.