When an agent streams text and then immediately runs tool calls, the
webchat UI drops the streamed content: the "final" event arrives with
message: undefined (buffer consumed by sub-run), and the client clears
chatStream without saving it to chatMessages.
Before clearing chatStream on a "final" event, check whether the stream
buffer has content. If no finalMessage was provided but the stream is
non-empty, synthesize an assistant message from the buffered text —
mirroring the existing "aborted" handler's preservation logic.
Closes#31895
15226b0b83