Replace assert False with KeyError in DAGCircuit methods

[ddri]

Summary

• Replace assert False with proper KeyError exceptions in next_element() and prev_element()
• Add docstrings for both methods (previously marked # DOCME TESTME)

Problem

The methods used assert False as error handling:

def next_element(self, elem: Operation, qubit: Optional[Qubit] = None) -> Operation:
    for _, node, key in self.graph.edges(elem, keys=True):
        if qubit is None or key == qubit:
            return node
    assert False  # Insanity check  # FIXME, raise exception

This is problematic because:

1. Running Python with -O disables assertions, causing the function to return None
2. AssertionError with no message provides no diagnostic information

Fix

Now raises KeyError with descriptive message:

raise KeyError(f"No next element found for {elem!r} on qubit {qubit}")

Test plan

• Verified fix manually - KeyError raised with helpful message
• All existing dagcircuit_test.py tests pass (including test_next_prev)

Fixes #130