runner.go: Merge partial unicode characters before sending

We check for partial unicode characters and accumulate them before
sending. However, when we did send, we still sent each individual piece
separately, leading to broken output. This combines everything into
a single group, which is also more efficient.

This also switches to the built-in check for valid unicode characters,
which is stricter. After this, we should never send back an invalid
sequence.

Fixes #7290

integration/basic_test.go

@@ -30,6 +30,22 @@ func TestOrcaMiniBlueSky(t *testing.T) {
 	GenerateTestHelper(ctx, t, req, []string{"rayleigh", "scattering"})
 }
 
+func TestUnicodeOutput(t *testing.T) {
+	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Minute)
+	defer cancel()
+	// Set up the test data
+	req := api.GenerateRequest{
+		Model:  "gemma2:2b",
+		Prompt: "Output some smily face emoji",
+		Stream: &stream,
+		Options: map[string]interface{}{
+			"temperature": 0,
+			"seed":        123,
+		},
+	}
+	GenerateTestHelper(ctx, t, req, []string{"😀", "😊", "😁", "😂", "😄", "😃"})
+}
+
 func TestUnicodeModelDir(t *testing.T) {
 	// This is only useful for Windows with utf-16 characters, so skip this test for other platforms
 	if runtime.GOOS != "windows" {