Streaming fix (again) for sending final even when no text buffered

[tracyboehrer]

Fixes #310

See original notes in #313

[Copilot]

Pull Request Overview

Ensures the final streaming response always contains text even if no content was buffered.

• Use a conditional to assign a default message when Message is null or empty
• Preserve existing logic around Activity.Entities

Comments suppressed due to low confidence (1)

src/libraries/Builder/Microsoft.Agents.Builder/StreamingResponse.cs:319

• Add a unit test to verify that when Message is null or empty, CreateFinalMessage sets activity.Text to the fallback string. This will help prevent regressions.

activity.Text = !string.IsNullOrEmpty(Message) ? Message : "No text was streamed";   // Teams won't allow Activity.Text changes or empty text