Order-dependent test failures: custom_globals.lp mutates global Python evaluation environment (_shared_environment) and leaks across tests

Running the full test suite via nox -s test can produce order-dependent failures in tests that use Python operators like python("math.pow"). The root cause is that custom_globals.lp deliberately modifies the global/shared Python evaluation environment in constraint_handler.evaluator (specifically _shared_environment) and that change persists for the remainder of the pytest process.

Because the Python helper code used by @pythonEvalExpr / Python operators reads globals from constraint_handler.evaluator._shared_environment, mutating it in one test affects unrelated subsequent tests. This makes the suite sensitive to test order and can cause flaky failures.

Steps to reproduce

Run the full suite:
nox -s test
Observe that after custom_globals runs, a later test that relies on math being present in the Python environment may fail (e.g., a fixture using operation(python("math.pow"), ...)).

Related to #72