Refactor C++ sources: fix bugs, reduce duplication, improve consistency

[WillAyd]

• Fix log_config erasure bug: process_params.erase() was deleting user-provided log_config values instead of keeping them. Replaced with emplace() semantics.
• Extract MakeHyperProcess() helper into hyper_process.hpp to deduplicate HyperProcess setup across reader.cpp, writer.cpp, and libpantab.cpp.
• Fix throw "unreachable" (const char*) to throw std::logic_error, making exceptions catchable as std::exception.
• Fix typo "unknownn" -> "unknown" in error message and fix misleading "ArrowAppendString failed" in BytesReadHelper.
• Extract PrecisionLimit constant as kMaxNumericPrecision/kNumericVariantSize in numeric_gen.hpp, replacing 3 duplicated magic number definitions.
• Refactor ReadHelper to use template method pattern: null checking is now handled once in the base class Read() method, with subclasses overriding ReadNonNull() instead. Eliminates ~60 lines of duplicated null checks.
• Deduplicate timestamp switch in MakeInsertHelper via MakeTimestampInsertHelper template helper, reducing 8 near-identical branches to 4 one-line calls.

closes #xxx