refactor: modularize dict_to_protobuf and simplify field mapping to reduce complexity

[bhagathkrishnacdac]

Description
This PR refactors _get_field_mapping and _dict_to_protobuf in pybess/protobuf_to_dict.py to address code complexity and improve readability. The monolithic, highly nested structures have been decomposed into smaller, single-responsibility helper functions.

Key Changes

• Decomposed Field Mapping (_get_field_mapping):
  • Extracted _process_regular_fields to isolate standard key-value conversions.
  • Extracted _process_extension_fields to isolate protobuf extensions mapping.
  • Fixed an exception-formatting bug from the original codebase where the incorrect variable (key instead of ext_num) was referenced during a strict KeyError.
• Modularized Deserialization (_dict_to_protobuf):
  • Replaced deeply nested conditional trees with structured execution handlers (_handle_repeated_field, _handle_message_field, _handle_extension_field) that return boolean flags to coordinate loop progression.
  • Extracted dedicated handlers for map-fields (_handle_map_field) and array-fields (_handle_regular_repeated_field).
  • Preserved the exact original evaluation order: running type-callable mapping before checking extension assignments, and enum conversions after.
  • Cleaned up and removed the unused _convert_field_value helper to prevent dead code.