BUG: Measurement error assumes all qubits measured when no explicit measurement

[Manideep3969]

Issue

cost_model.py:376-378 — When a circuit has no measurement instructions, _get_measured_qubits returns set(range(circuit.num_qubits)), assuming all qubits are measured. This inflates measurement error for circuits that don't measure all qubits.

Fix

Return an empty set when no measurements are found, or make this behavior configurable.