Unexpected behavior in `BaseDomain.update` method

[GiovanniCanali]

Describe the bug
The BaseDomain.update method does not behave correctly when a variable changes type. Specifically, if a key is defined as a range variable in the original domain and as a fixed variable in the updating domain, the update does not properly override the original definition.

To Reproduce

domain1 = CartesianDomain({"x": [0, 1], "y": [0, 1], "t": 0})
domain2 = CartesianDomain({"x": 0, "z": 0})
new_domain = domain1.update(domain2)

print(new_domain.domain_dict)

Expected behavior
new_domain should reflect the values in domain2 when keys overlap. In particular, since "x" is defined as a fixed variable in domain2, it should override the ranged definition in domain1. Therefore, the expected domain is:

{"t": 0, "x": 0, "z": 0,  "y": [0, 1]}

Instead, "x" keeps its original ranged definition from domain1, and the output is:

{'t': 0, 'x': (0, 1), 'z': 0, 'y': (0, 1)}