MR !803 Thread #17: Export data classes and exceptions in public API

Add imports and exports for:
- Data classes: Accounts, Authority, AuthorityInfo, Balances, KeyPair, Witness, WitnessesResult
- Exceptions: AccountNotFoundError, InvalidAccountNameError, InvalidNumberOfKeyPairsError,
  InvalidPageNumberError, InvalidPageSizeError, InvalidProfileNameError,
  PasswordRequirementsNotMetError, PyContextManagerNotUsedError, PyError, PyValidationError

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