test: reduce boilerplate in imported-session tests
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- Merge the two strategy cleanup tests into one TestImportedSessions_SurviveCleanup with a shared saveImportedState helper.
- Merge the list/info read-only tests into one TestImportedSession_MarkedReadOnly with a run helper.
Co-Authored-By: Claude Opus 4.8 noreply@anthropic.com Entire-Checkpoint: 01KX4DTCMV8EMWKYCKMBASNVBA
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Checkpoint 1