Fix instruction duration unit in estimated success probability calculation

[Shaobo-Zhou]

Description

Fixes instruction duration unit in estimated success probability calculation in src/mqt/predictor/reward.py, as the default unit is "dt" instead of "s": https://github.com/Qiskit/qiskit/blob/stable/2.1/qiskit/transpiler/target.py#L415-L430

Checklist:

• The pull request only contains commits that are focused and relevant to this change.
• I have added appropriate tests that cover the new/changed functionality.
• I have updated the documentation to reflect these changes.
• I have added entries to the changelog for any noteworthy additions, changes, fixes, or removals.
• I have added migration instructions to the upgrade guide (if needed).
• The changes follow the project's style guidelines and introduce no new warnings.
• The changes are fully tested and pass the CI checks.
• I have reviewed my own code changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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