[codex] Add regression test for barrier recombination in separate_circuit

[TSS99]

Summary

• add a regression test for separate_circuit when multiple qubits remain in the same partition after barrier splitting
• assert that _combine_barriers rebuilds shared barriers instead of leaving duplicated single-qubit barriers behind

Root cause

separate_circuit relies on _split_barriers before partitioning, then _combine_barriers when reconstructing each subcircuit. The existing barrier coverage only exercised one-qubit subcircuits, so the suite still passed even if _combine_barriers was removed.

Fix

• extend test_separate_circuit with a shared-partition barrier case using partition labels "AAB"
• verify the resulting subcircuit contains two-qubit barriers in the shared partition, not separate one-qubit barriers

Tests

• python -m tox -e py311 -- test/utils/test_transforms.py
• python -m tox -e lint
• temporary mutation check: commenting out _combine_barriers(tmp_circ) makes test/utils/test_transforms.py fail in the new subtest

Notes or limitations

• this change is intentionally test-only; it does not modify runtime behavior

Fixes #626

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