Python: chtypes.errors exports internal and misuse helpers through __all__ #499

Found by the public-surface parity gate (#436).

• What: chtypes.errors.__all__ lists internal and misuse, which are construction helpers, not part of the documented error surface (docs/reference/bindings-v1.md §4).
• Effect: they become public API by the __all__ convention, from chtypes.errors import * pulls them in, and no other binding has them.
• Fix: drop them from __all__ (an underscore name, or an internal module), then remove the gate's allowlist entries for them.
• Compatibility: removing a name from __all__ is technically visible. Check that nothing documented uses them, and record it in the CHANGELOG.

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