Incorporates changes from XRPLF/rippled PR #5468:
- Add getBlockSize() method to Backend interface
Returns std::optional<std::size_t> for backends that support
configurable block sizes (currently only NuDB)
- Update parseBlockSize() to throw exceptions instead of warnings
Invalid block size configurations now throw std::runtime_error
with descriptive error messages instead of silently using defaults
- Read existing database block size as default
parseBlockSize() now checks existing nudb.key file for block size
using nudb::block_size() before applying configuration
- Add comprehensive unit tests (NuDBFactory_test.cpp)
Tests cover default sizes, valid sizes (4K-32K power-of-2),
invalid sizes, error messages, power-of-2 validation, and
data persistence across different block sizes
These changes improve configurability and error reporting while
maintaining backward compatibility.
