Apply remaining PY layer code review fixes

This commit includes various improvements from code review:

- ShowBalances: Add account existence check before fetching data
- ShowAccounts: Use tuple instead of list for accounts
- ShowWitnesses: Add docstring, use tuple, update votes mapping
- ShowAuthority: Add VALID_AUTHORITIES constant, add validation, use tuple
- ShowInterface: Use composition instead of inheritance (LSP fix),
  access world via property instead of _world
- Exceptions: Improve InvalidAccountNameError with description parameter,
  add max bounds for page number and page size validators
- Validators: Add MAX bounds, fix bool check for int validation
- WitnessData: Add votes_raw attribute for raw vote count
- Add functional tests for exceptions and validators

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