mcp: add CallToolResult.getError (#481)

Provide a way for middleware to get the error from a tool call,
demonstrating that we can add this functionality without making a
breaking change.

Leave getError unexported for now; we can export it (and setError) at
any time.

For #64.

mcp/streamable_test.go

@@ -159,7 +159,7 @@ func TestStreamableTransports(t *testing.T) {
 			want := &CallToolResult{
 				Content: []Content{&TextContent{Text: "hi foo"}},
 			}
-			if diff := cmp.Diff(want, got); diff != "" {
+			if diff := cmp.Diff(want, got, ctrCmpOpts...); diff != "" {
 				t.Errorf("CallTool() returned unexpected content (-want +got):\n%s", diff)
 			}
 
@@ -550,8 +550,6 @@ func resp(id int64, result any, err error) *jsonrpc.Response {
 	}
 }
 
-var ()
-
 func TestStreamableServerTransport(t *testing.T) {
 	// This test checks detailed behavior of the streamable server transport, by
 	// faking the behavior of a streamable client using a sequence of HTTP