input: Write a Python function that accepts a string consisting of space-separated lowercase letters and returns a dictionary containing the most frequently occurring letters along with their counts. If multiple letters have the same highest frequency, include all of them.
  assert frequency_dict('a b b a') == {'a': 2, 'b': 2}
  assert frequency_dict('a b c a b') == {'a': 2, 'b': 2}
  assert frequency_dict('a b c d g') == {'a': 1, 'b': 1, 'c': 1, 'd': 1, 'g': 1}
  assert frequency_dict('r t g') == {'r': 1, 't': 1, 'g': 1}
  assert frequency_dict('b b b b a') == {'b': 4}
  assert frequency_dict('r t g') == {'r': 1, 't': 1, 'g': 1}
  assert frequency_dict('') == {}
  assert frequency_dict('a') == {'a': 1}
output: def frequency_dict(s):
      from collections import Counter
      if not s:
          return {}
      
      words = s.split()
      counts = Counter(words)
      max_count = max(counts.values())
      
      result = {k: v for k, v in counts.items() if v == max_count}
      
      return result