Remove separate BigNumberCell thrift struct and use a unified NumberCell
with an optional string_value field for precision. This simplifies the
data model while preserving bigint precision support.

Changes:
- Add string_value field to NumberCell in thrift
- Remove BigNumberCell struct and big_number_cell from Cell union
- Update mapData() to always create number_cell with optional string_value
- Simplify renderer NumberCell class to read directly from cell
- Update test utilities to check string_value instead of cell kind