.NET: [Bug]: Agent chat completions endpoint does not terminate SSE stream with data: [DONE]\n\n

@westey-m is already working on this.

Since Sep 18, 2026.

### Description

- The agent chat completions endpoint does not reliably write a data: [DONE]\n\n event at the end of the SSE stream. The stream closes without the terminator event that OpenAI-compatible clients expect.
- The SSE stream should terminate with data: [DONE]\n\n, consistent with the OpenAI SSE specification. Clients that rely on the [DONE] signal to detect stream completion will hang or timeout.
- Steps to reproduce the issue:
- Register an agent chat completions endpoint
- Send a request that produces a streaming SSE response
- Observe that the stream closes without a final data: [DONE]\n\n event
- We implemented an opt-in middleware that wraps the response body and appends data: [DONE]\n\n if the endpoint didn't already write it.

### Code Sample

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### Error Messages / Stack Traces

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### Package Versions

Microsoft.Agents.AI 1.17.0; Microsoft.Agents.AI.Hosting.OpenAI 1.17.0-alpha.260804.1; Microsoft.Agents.AI.Hosting.AGUI.AspNetCore 1.17.0-preview.260804.1

### .NET Version

NET 10.0

### Additional Context

This requires consumers to implement custom middleware or post-processing to detect stream end, which is fragile. The chat completions endpoint should terminate its SSE stream with data: [DONE]\n\n per the OpenAI SSE convention.