feat: handle edge cases in scval decoder and add tests

[Agbasimere]

Description

The argument decoder (scval_to_string) was silently ignoring complex ScVal variants, producing missing or incorrect outputs. This PR updates the decoder to accurately translate the remaining variants into readable string representations.

Changes Made

• Added handling for ScVal::Void and empty vectors to represent empty arguments correctly.
• Added handling for ScVal::U128 and ScVal::I128 to correctly decode max-size integers.
• Added recursive decoding support for ScVal::Vec and ScVal::Map, allowing for nested mappings inside vectors.
• Introduced a full unit test suite within the decode::diagnostic module to ensure all edge cases are strictly covered and verified.

Testing

• Unit tests added for:

  • Empty args (ScVal::Void, empty ScVec)
  • Large integers (Max/Negative 128-bit parsing)
  • Nested types (an ScMap inside an ScVec)

  Closes Write Unit Tests That Cover Every SCVal Variant the Argument Decoder Handles #244