Fix CSPLayout to detect 3-or-more-qubit gates (fix #7155)

[hijirii]

Summary

CSPLayout was only checking 2-qubit gates (via dag.two_qubit_ops()) and ignoring gates with 3+ qubits like CCX. This caused the pass to incorrectly report 'solution found' for circuits with multi-qubit gates that cannot be mapped to a linear coupling map.

Fix

Added a check for any gate with more than 2 qubits before running the CSP solver. If such a gate is found, the pass now fails gracefully with CSPLayout_stop_reason = "3-or-more-qubit gate found".

Changes

• csp_layout.py: Added early-exit check for 3+ qubit gates
• test_csp_layout.py: Added test case covering CCX on linear coupling map

Reproduction (before fix)

from qiskit import QuantumCircuit
from qiskit.transpiler import CouplingMap
from qiskit.transpiler.passes import CSPLayout

qc = QuantumCircuit(3)
qc.cx(0, 1)
qc.ccx(2, 0, 1)

pass_ = CSPLayout(CouplingMap([[0, 1], [1, 2]]))
result = pass_(qc)
print(pass_.property_set["CSPLayout_stop_reason"])
# Before: 'solution found' (WRONG - CCX needs complete connectivity)
# After:  '3-or-more-qubit gate found'

Checklist

• Tests pass (existing + new test for this case)
• Follows Qiskit coding style
• Documentation updated (if needed)
• Changelog entry (if applicable)

Closes #7155

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