Cannot modify constraints after adding to problem

Before the rust rewrite, this code modified the constraint within the problem:

```py
>>> import pulp as lp
>>> prob = lp.LpProblem()
>>> x = lp.LpVariable('x')
>>> constr = x <= 2
>>> prob += constr
>>> constr[x] = 2
>>> constr
2*x + -2 <= 0
>>> prob.constraints
OrderedDict({'_C1': 2*x + -2 <= 0}
```

In the new version, the constraint doesn't get updated:

```py
>>> import pulp as lp
>>> prob = lp.LpProblem()
>>> x = prob.add_variable('x')
>>> constr = x <= 2
>>> prob += constr
>>> constr[x] = 2
>>> constr
2.0*x + -2.0 <= 0
>>> prob.constraints()
[1.0*x + -2.0 <= 0]
```

It is because the constraints are no longer stored as the given Python objects, but are copied into inmutable proxies instead:
```py
# Before
>>> prob.constraints['_C1'] is constr
True

# After
>>> prob.constraints()[0] is constr
False
```

Start by running the reported Python reproduction through LpProblem, add_variable, prob +=, and constraint mutation, then inspect how constraints are stored and returned after the Rust rewrite. Done means mutating the original constraint after adding it updates the problem's stored constraint, with a regression test covering that behavior.

Written by the indexing model from the issue text.