refactor(network): use getPackedByteCount() in NetPacket.cpp

Refactor NetPacket.cpp to use the new getPackedByteCount() polymorphic
interface instead of manual size calculations.

Changes:
- Replace large switch statement in GetGameCommandSize() with single
  call to msg->getPackedByteCount()
- Simplify size calculations throughout by using getPackedByteCount()
- Replace C-style casts with static_cast for type safety
- Remove redundant manual size computations

Author: bobtista