[Bug]: ctx.stream.close() returns placeholder activity ID

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### Bug Description

`await ctx.stream.close()` returns a `SentActivity` whose `id` is
`DO_NOT_USE_PLACEHOLDER_ID` instead of the ID of the streamed activity.

Teams returns the stream/activity ID from the initial streaming request. Later streaming requests, including the final request, return an empty successful response. The SDK currently maps the empty response to its placeholder ID and returns that value from `HttpStream.close()`, even though the stream has stored the actual ID from the initial request.

### Steps to Reproduce

1. Emit content through `ctx.stream.emit()`.
2. Await the activity with `ctx.stream.close()`.
3. Observe that `result.id` is `DO_NOT_USE_PLACEHOLDER_ID`.

### Expected Behavior

`ctx.stream.close()` should return a `SentActivity` with the original stream ID, or at least not the `DO_NOT_USE_PLACEHOLDER_ID` value.

### Actual Behavior

`ctx.stream.close()` returns a `SentActivity` set to `DO_NOT_USE_PLACEHOLDER_ID`.

### SDK Version

v2.0.0a31

### Python Version

3.12.10

### Additional Context

_No response_

Start at the HttpStream.close() entry point and trace how the stored stream ID and the empty final response are mapped into the returned SentActivity. Reproduce the issue with ctx.stream.emit() followed by ctx.stream.close(), then verify that the result uses the original stream ID rather than DO_NOT_USE_PLACEHOLDER_ID.

Written by the indexing model from the issue text.