fix(chat): NewStreamerHub accepts ConversationService interface; SSE handler drains ready events before ctx

internal/chat/sse_test.go

@@ -183,8 +183,13 @@ func TestStreamMessages_StopsWhenClientClosesContext(t *testing.T) {
 		r.ServeHTTP(w, req)
 	}()
 
-	// Send one event, then cancel.
+	// Send one event and give the handler a moment to drain it before cancelling,
+	// so the body assertion is deterministic. The cancel itself is what we're
+	// verifying — that the handler exits even though the channel never closes.
 	ch <- SSEEvent{ID: 1, Event: "text_delta", Data: "hi"}
+	require.Eventually(t, func() bool {
+		return strings.Contains(w.Body.String(), "text_delta")
+	}, time.Second, 10*time.Millisecond, "event was not flushed before cancel")
 	cancel()
 
 	// Handler must return promptly after context cancellation.
@@ -195,7 +200,6 @@ func TestStreamMessages_StopsWhenClientClosesContext(t *testing.T) {
 		t.Fatal("handler did not stop after context cancellation")
 	}
 
-	// The one event before cancel should have been written.
 	assert.Contains(t, w.Body.String(), "text_delta")
 }