Fix QASM3 experimental delay unit serialization

[kelvinvelasquez-SDE]

Summary

• fix the DurationUnit::Millisecond serializer so experimental QASM3 exports emit ms instead of us
• correct the picosecond-to-nanosecond conversion path used by qasm3.dumps_experimental
• add a focused regression test covering ms and ps delay serialization

Test plan

• Run python -m unittest test.python.qasm3.test_export.TestQASM3ExporterRust.test_delay_units test.python.qasm3.test_export.TestQASM3ExporterRust.test_delay_qpy_roundtrip test.python.qasm3.test_export.TestCircuitQASM3.test_delay_statement in an isolated validation worktree

fixes #16097

AI assistance disclosure: GPT-5.4 (Cursor)

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[1ucian0]

I'm closing this one in favor of #16114, which landed first. Feel free to take any issue from qisk.it/good-first-issues for other first-time-contributor-friendly issues . Thanks!