Fix: Remove macros from namespace when deleted

[Copilot]

When deleting a macro via del model.macro_name, it was removed from _macros dict but persisted in _macro_namespace, causing stale references.

Changes

• modelx/core/model.py: Modified del_macro() to also remove the macro from _macro_namespace when present
• modelx/tests/core/model/test_macro.py: Updated test_macro_delete() to verify namespace cleanup

Example

import modelx as mx

m = mx.new_model('MyModel')

@mx.defmacro
def get_model_name():
    return mx_model._name

m.get_model_name()  # Creates namespace
del m.get_model_name

# Now passes (previously failed)
assert 'get_model_name' not in m._impl._macro_namespace

The fix handles the lazy initialization of _macro_namespace (only created on first macro execution) by checking for None before attempting deletion.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Bug: Macros not removed from the namespace when they are removed</issue_title>
<issue_description>Fix the bug that macros are not removed from Model._macro_namespace when they are deleted.

import modelx as mx

m = mx.new_model('MyModel')

@mx.defmacro
def get_model_name():
    return mx_model._name

m.get_model_name()

del m.get_model_name

assert 'get_model_name' not in m._impl._macro_namespace   # This should pass but it raises AssertionError
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes Bug: Macros not removed from the namespace when they are removed #208

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