Fix sendError message's format

src/cdp/cdp.zig

@@ -733,8 +733,7 @@ pub fn Command(comptime CDP_T: type, comptime Sender: type) type {
         pub fn sendError(self: *Self, code: i32, message: []const u8) !void {
             return self.sender.sendJSON(.{
                 .id = self.input.id,
-                .code = code,
-                .message = message,
+                .@"error" = .{ .code = code, .message = message },
             });
         }
 

src/cdp/testing.zig

@@ -180,8 +180,7 @@ const TestContext = struct {
     pub fn expectSentError(self: *TestContext, code: i32, message: []const u8, opts: ExpectErrorOpts) !void {
         const expected_message = .{
             .id = opts.id,
-            .code = code,
-            .message = message,
+            .@"error" = .{ .code = code, .message = message },
         };
         try self.expectSent(expected_message, .{ .index = opts.index });
     }