Implement auto-select logic for modify/remove transfer-schedule

Add auto-selection for single transfer with pair_id=0:
- validate_pair_id_should_be_given() checks if --pair-id is needed
- _identity_check() auto-selects when conditions are met
- Move validations to fetch_data() to run before _create_operations()

Fix CLITester: pair_id parameter now accepts None.

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