BlockKit 2 fails to validate StaticSelect initial_option values with grouped options

[nickovs]

Slack supports StaticSelect menus with grouped options, so that you can separate the options into related sets. It can also support specifying an initially selected option. Slack lets you do both at the same time. BlockKit v1 let you do both at the same time. BlockKit v2 fails when you try to do this while validating the option selections, raising the error:

blockkit.core.ComponentValidationError: Component 'StaticSelect': 'initial_option' has items that aren't present in the 'option_groups' 

The problem seems to stem from the use of Within validators attached to the StaticSelect class (the MultiStaticSelect has the same issue). The value for the initial_option is a single option (for initial_options for the multi-select it's a flat list of options) and you are comparing it to a list of groups, not a list of options.

The fix should be in the Within validator. When the target field contains OptionGroup objects, it should collect the options from within them and check if the initial value is a subset of the flattened set of values within the groups, rather than comparing against the groups themselves (which will always fail).

blockkit_bug_demo.py

[nickovs]

Inserting something like this into the middle of `Within.validate() should do the trick:

          # If the target contains OptionGroups, flatten to the                                                                                                                                                    
          # options within them                                                                                                                                                                                    
          flattened = []                                                                                                                                                                                           
          for item in target_value:                                                                                                                                                                                
              if isinstance(item, OptionGroup):                                                                                                                                                                    
                  group_opts = item._get_field_value("options")                                                                                                                                                    
                  if group_opts:                                                                                                                                                                                   
                      flattened.extend(group_opts)                                                                                                                                                                 
              else:                                                                                                                                                                                                
                  flattened.append(item)                                                                                                                                                                           
          target_value = flattened                                                                                                                                                                                 

[nickovs]

PR #131 is a fix for this.