[BUG] "unregistered" program type still included in `ConversionGraph`

[WingCode]

Copied from qBraid/qBraid #789

After calling unregister_program_type("qasm2"), the program type "qasm2" is removed from QPROGRAM_REGISTRY, but it is still included in the ConversionGraph. This leads to inconsistent behavior where "qasm2" is no longer recognized in the registry but still exists as a node in a new instance of ConversionGraph.

To reproduce:

from qbraid import QPROGRAM_REGISTRY, ConversionGraph, unregister_program_type

print("qasm2" in QPROGRAM_REGISTRY)  # True

graph = ConversionGraph()

print(graph.has_node("qasm2"))  # True

unregister_program_type("qasm2")

print("qasm2" in QPROGRAM_REGISTRY)  # False

new_graph = ConversionGraph()

print(new_graph.has_node("qasm2"))  # True, but should be False

After unregistering "qasm2", it should not be present in new instances of ConversionGraph. It should be a requirement that all "nodes" in the ConversionGraph are "registered" program types.

Note: The "qasm2" alias was just used as an example. This issue is applicable to all program types. The solution you propose should generalize to all program types.