🐛 Preserve circuit names during mapping

[burgholzer]

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Description

This is the base of a two-PR stack; #1153 should be merged after this PR.

Copy the input circuit name to the mapped circuit in the shared mapper initialization path. This preserves QuantumCircuit.name across both heuristic and exact mapping and avoids downstream rejection of unnamed circuits.

Add an exact-mapper regression test that checks the returned Qiskit circuit retains the input name.

AI assistance disclosure: GPT-5 via Codex traced the shared mapper initialization, implemented the one-line fix and focused test, and ran the reported validation.

Fixes #1151

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.
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• 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.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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