chore: python bindings cleanup and bit of refactor

[fresh-borzoni]

Summary

Refactor LogScanner to use ScannerKind enum

Replace two Option<T> fields (inner, inner_batch) on LogScanner with a
single ScannerKind enum. The previous design allowed impossible states, both
fields None, or both Some — which required defensive if/else chains in
every method. The enum makes the active scanner variant explicit at the type
level.

[fresh-borzoni]

@leekeiabstraction @luoyuxia PTAL 🙏