`StreamableClient` tries to JSON-decode ping message

[ccreutzi]

Describe the bug
A clear and concise description of what the bug is.

If an SSE server response does not start with a message event, but a ping event, the streamable client will try to decode the ping data as JSON and fail.

I have observed this on some calls to the DeepWiki MCP server.

To Reproduce
Steps to reproduce the behaviour:

func TestStreamableClientWithDelayedResponse(t *testing.T) {
	// This test simulates a server that takes a while to respond and sends a ping before its message
	ctx := context.Background()
	fake := &fakeStreamableServer{
	t: t,
	responses: fakeResponses{
		{"POST", "", methodInitialize}: {
			header: header{
				"Content-Type":  "application/json",
				sessionIDHeader: "123",
			},
			body: jsonBody(t, initResp),
		},
		{"POST", "123", notificationInitialized}: {
			status:              http.StatusAccepted,
			wantProtocolVersion: latestProtocolVersion,
		},
		{"GET", "123", ""}: {
			header: header{
				"Content-Type": "text/event-stream",
			},
			wantProtocolVersion: latestProtocolVersion,
		},
		{"POST", "123", methodCallTool}: {
			header: header{
				"Content-Type": "text/event-stream",
			},
			wantProtocolVersion: latestProtocolVersion,
			body: `event: ping
data: ping

event: message
data: {"jsonrpc":"2.0","id":2,"result":{"content":[{"type":"text","text":"tool result"}]}}

event: ping
data: ping
`,
			},
			{"DELETE", "123", ""}: {},
		},
	}

	httpServer := httptest.NewServer(fake)
	defer httpServer.Close()

	transport := &StreamableClientTransport{Endpoint: httpServer.URL}
	client := NewClient(testImpl, nil)
	session, err := client.Connect(ctx, transport, nil)
	if err != nil {
		t.Fatalf("client.Connect() failed: %v", err)
	}

	toolAndArgs := &CallToolParams{Name: "tool", Arguments: map[string]interface{}{}}
	result, err := session.CallTool(ctx, toolAndArgs)
	if err != nil {
		t.Fatalf("client.CallTool() failed: %v", err)
	}
	expected := "tool result"
	if result == nil || len(result.Content) == 0 {
		t.Errorf("CallTool result is nil or empty")
	}
	if result.Content[0].(*TextContent).Text != expected {
		t.Errorf("CallTool result: got %q, want %q", result.Content[0].(*TextContent).Text, expected)
	}

	if err := session.Close(); err != nil {
		t.Errorf("closing session: %v", err)
	}

}

Expected behavior
A clear and concise description of what you expected to happen.

The logic taking apart an event stream should skip ping events and only tro to decode message events.

[findleyr]

Thanks, I believe this is indeed a bug (probably in the logic that peeks at messages beginning the stream.
I will fix.