fix: accept metadata-only SSE chunks instead of aborting stream in StreamingResponseAggregator

[kdroste-google]

Modified version of #783

Instead of ignoring the SSE chunks with no candidates, this version accepts them and passes them downstream to further processing (they contain usageMetadata)

[kdroste-google]

Manual tests:
Replace quickstart main with

func main() {
	ctx := context.Background()

	model, err := gemini.NewModel(ctx, "gemini-3.1-flash-lite", &genai.ClientConfig{
		Backend:  genai.BackendVertexAI,
		Project:  "PROJECT",
		Location: "global",
	})
	if err != nil {
		log.Fatalf("Failed to create model: %v", err)
	}

	a, err := llmagent.New(llmagent.Config{
		Name:        "weather_time_agent",
		Model:       model,
		Description: "Helpful Agent",
		Instruction: "Your purpose is to help the user by using search tool.",
		Tools: []tool.Tool{
			geminitool.GoogleSearch{},
		},
	})
	if err != nil {
		log.Fatalf("Failed to create agent: %v", err)
	}

	config := &launcher.Config{
		AgentLoader: agent.NewSingleLoader(a),
	}

	l := full.NewLauncher()
	if err = l.Execute(ctx, config, os.Args[1:]); err != nil {
		log.Fatalf("Run failed: %v\n\n%s", err, l.CommandLineSyntax())
	}
}

Execute:

echo "What are some recent movie recommendations?" | go run ./examples/quickstart/main.go  --streaming_mode sse

[kdroste-google]

@anishesg , please be aware, basing on your PR I have created a new one (with your contributions included), solving the issue in a different way.