fix: preserve gate labels through transpilation

[elkadylojain-ops]

Gate labels were silently dropped or only partially propagated when a labeled gate was decomposed into multiple gates during transpilation.

Before this fix:

circuit = QuantumCircuit(2)
circuit.cx(0, 1, label='hello')
w = transpile(circuit, basis_gates=['cz', 'rx', 'rz'])
for inst in w.data:
    print(inst.operation.name, inst.operation.label)
# rz   None
# rx   None
# cz   hello
# rx   hello
# rz   hello

After this fix:

# rz   hello
# rx   hello
# cz   hello
# rx   hello
# rz   hello

Four separate places were dropping the label when emitting new instructions to replace a labeled run/block:

• BasisTranslator: the label was only forwarded in the branch handling gates without parameters, not in the branch handling parameterized gates.
• ConsolidateBlocks: labels were dropped when consolidating a block or a 1Q run into a UnitaryGate.
• CommutativeCancellation and Optimize1qGatesDecomposition: both call DAGCircuit::insert_1q_on_incoming_qubit, which unconditionally set label: None. Added insert_1q_on_incoming_qubit_with_label so callers can propagate the original label.
• TwoQubitPeepholeOptimization: same issue when re-emitting a synthesized 2Q gate sequence.

Fixes #15168

AI/LLM disclosure

• I didn't use LLM tooling, or only used it privately.
• [ x] I used the following tool to help write this PR description: Claude (Anthropic)
• I used the following tool to generate or modify code:

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