Allow Text to be change for final StreamingResponse message

[tracyboehrer]

Currently, the complete streamed buffer is always sent in the final message.

[MattB-msft]

Connecting with teams Streaming API folks to verify behavior.

[tracyboehrer]

Confirmed that changing the final Activity.Text will cause an error on the Teams side (BadRequest because streamed content changed). Meeting next week to detail other Teams rules around this.

[tracyboehrer]

Closing. Teams does not allow text changes. But the real scenario for this was to support AI Citations, which we added official support for in another PR.