traitlets Dict does not observe value changes after initial key / value assigments

I'm using `traitlets` version 5.14.3 and Python 3.12.3...

In both cases below, a `TraitError` is not thrown, even though `None` is assigned to a `Unicode()` field after initialization...

```python
from traitlets import HasTraits, Dict, Unicode

class Foo(HasTraits):
data = Dict(per_key_traits={"a_key": Unicode()})

class Bar(HasTraits):
data = Dict(key_trait=Unicode(), value_trait=Unicode())

foo = Foo()
foo.data = {"a_key": "a_value"}
# I think this should throw a TraitError...
foo.data["a_key"] = None

bar = Bar()
bar.data = {"a_key": "a_value"}
# I think this should throw a TraitError...
bar.data["a_key"] = None
```

Start by running the two Python reproductions in the issue against traitlets 5.14.3, then trace Dict validation for mutations after initial assignment. Done means both invalid value assignments raise TraitError and regression coverage verifies the per-key and key/value trait cases.

Written by the indexing model from the issue text.