preserve integer width in JsonExporter::fromJson

[aysha-afrah26]

ExportBlackboardToJSON writes int64_t and uint64_t entries out at full width, but fromJson read every JSON integer back with nlohmann's get(), which is just a static_cast of the stored 64-bit value with no range check. Anything outside the int32 range wraps silently, so a saved int64_t of 5000000000 comes back as 705032704 and 4294967296 comes back as 0, and the same happens to any value fed through ImportBlackboardFromJSON or ImportTreeFromJSON when restoring tree state. I hit it doing an export/import round-trip and noticing the restored counter no longer matched. The fix keeps the width during import: values that fit in int stay int, larger magnitudes stay uint64_t or int64_t to match what toJson already emits. Small integers are unaffected, so the common case behaves exactly as before. Added a regression test that imports the out-of-range values and checks the round-trip.