Use FinishReasonNull for EOF fallthrough with no output

When the stream ends via EOF without an explicit finish reason and
produces no content or tool calls, use FinishReasonNull instead of
FinishReasonStop. The provider never signalled natural completion,
so claiming stop would give clients a false signal.

Author: trungutt

pkg/runtime/streaming.go

@@ -196,7 +196,10 @@ func (r *LocalRuntime) handleStream(ctx context.Context, stream chat.MessageStre
 
 	finishReason := chat.FinishReasonToolCalls
 	if stoppedDueToNoOutput {
-		finishReason = chat.FinishReasonStop
+		// No finish reason was received from the provider and the stream
+		// produced nothing.  Use FinishReasonNull rather than FinishReasonStop
+		// so clients don't mistake this for a natural completion.
+		finishReason = chat.FinishReasonNull
 	}
 
 	return streamResult{