Add unit tests for transfer-schedule validation logic

Test validate_pair_id_should_be_given() for all scenarios:
- pair_id specified: always passes
- single transfer with pair_id=0: auto-select OK
- single transfer with pair_id!=0: error with hint
- multiple transfers: error requiring explicit pair_id
- no transfers: passes (fails later on existence check)

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>