[matter_yamltests] Rewrite as_mapping to be more readable

[vivien-apple]

          ok, but why are we doing that? What is the purpose of having a separate item without `self` just to re-assign to self at the end of the method?

readability-wise this is a very long method, effectively what we seem to want is:

def _get_mappings(self, test, definitions) -> Mappings
   if definitions is None or not definitions.has_cluster_by_name(cluster_name):
      return Mappings(None,None,None)
   if self.is_attribute:
      return self.get_attribute_mappings(test, definitions)
   if self.is_event:
      return self.get_event_mappings(test, definitions)
   
   # assume we are a command (can this be checked?)
   return self.get_command_mappings(test, definitions)

def get_attribute_mappings(self, test, definitions):
    # .....
    return Mappings(arguments=..., response=..., name=...)

#...
def _update_mappings(self, test, definitions):
   mappings = self._get_mappings(test, definitions)
   
   # TODO: if we have a mappings dataclass, may as well not have 3 self variables
   #              and just have self.mappings ....
   self.argument_mapping = mappings.arguments
   self.response_mapping = mappings.response
   self.response_mapping_name = mappings.name

Originally posted by @andy31415 in #25109 (comment)

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