Fix missing f-string prefix in Circuit error message

[ddri]

Summary

• Fix missing f prefix on second line of error message string
• Add missing space before "but"

Problem

The error message for incommensurate qubits showed literal {list(qbs)} instead of the actual qubit list:

ValueError: Incommensurate qubits: Expected [0]but received {list(qbs)}

Fix

Added f prefix and space:

raise ValueError(
    f"Incommensurate qubits: Expected {list(qubits)} "
    f"but received {list(qbs)}"
)

Now produces:

ValueError: Incommensurate qubits: Expected [0] but received [0, 1]

Test plan

• Verified fix manually
• All existing circuits_test.py tests pass

Fixes #128