feat(State): Implement __delitem__ and pop methods for element removal

Why?

The State class is designed to manage state and should mimic the behavior of a standard Python dictionary (dict). However, it previously lacked standard methods for element removal, such as del state[key] and state.pop(key).

This PR addresses this gap by making the State class more convenient and consistent with standard Python conventions.

What does this change do? (Implementation Details)

This PR introduces two standard dictionary-like removal methods to the State class:

1. Implemented __delitem__(self, key):

   • Enables the use of the del state[key] syntax.
   • Correctly removes the key from both internal dictionaries (self._value and self._delta) to ensure state consistency.
   • Raises a KeyError if the key does not exist, fully replicating the behavior of a standard dict.

2. Implemented pop(self, key, default=...):

   • Adds the standard .pop() method, which removes a key and returns its corresponding value.
   • Supports an optional default argument to prevent a KeyError when the key is not found.
   • The implementation elegantly reuses the existing __getitem__ and __delitem__.

How to use it? (Usage Examples)

# Initial state
state = State(
    value={'user:name': 'Alice', 'user:role': 'admin'}, 
    delta={'user:role': 'editor', 'app:theme': 'dark'}
)
# state.to_dict() would return: {'user:name': 'Alice', 'user:role': 'editor', 'app:theme': 'dark'}

# 1. Deleting with `del`
del state['app:theme']
# The key 'app:theme' is now removed from the state.

# 2. Deleting with pop (when the key exists)
# The __getitem__ method will retrieve the value from delta ('editor')
removed_role = state.pop('user:role')
print(removed_role)
# Output: 'editor'

# 3. Deleting with pop and a default value (when the key does not exist)
status = state.pop('user:status', 'active')
print(status)
# Output: 'active'
# No error is raised.

# 4. Attempting to delete a non-existent key will raise an error (just like in a dict)
try:
    del state['non_existent']
except KeyError:
    print("KeyError, as expected!")

Unit-tests have been passed
Code auto-format has been passed
Tested on my own and works perfectly. Small but useful addition for more flexible state management