Fix () type mapping from SC_SPEC_TYPE_TUPLE to SC_SPEC_TYPE_VOID in contract specifications

[Copilot]

The Rust unit type () was incorrectly being encoded as SC_SPEC_TYPE_TUPLE instead of SC_SPEC_TYPE_VOID in contract specifications. This affected both function parameters and return types that use the unit type.

Problem

When defining a contract function like:

#[contractimpl]
impl Contract {
    pub fn void_fn(env: Env, void_argument: ()) -> () {
        void_argument
    }
}

The generated contract specification would incorrectly show:

• void_argument parameter as SC_SPEC_TYPE_TUPLE (empty tuple)
• Return type as SC_SPEC_TYPE_TUPLE (empty tuple)

Instead of the expected:

• void_argument parameter as SC_SPEC_TYPE_VOID
• Return type as SC_SPEC_TYPE_VOID

Solution

Modified the map_type function in soroban-sdk-macros/src/map_type.rs to handle empty tuples as a special case. The fix adds a simple check for empty tuples and returns ScSpecTypeDef::Void instead of creating an empty ScSpecTypeTuple:

Type::Tuple(TypeTuple { elems, .. }) => {
    // Handle unit type () as ScSpecTypeDef::Void
    if elems.is_empty() {
        return Ok(ScSpecTypeDef::Void);
    }
    // ... existing tuple handling logic for non-empty tuples
}

Testing

• Added comprehensive unit tests to verify the fix
• test_unit_type_should_be_void() confirms () maps to ScSpecTypeDef::Void
• test_non_empty_tuple_should_be_tuple() ensures non-empty tuples still work correctly
• All existing tests continue to pass (8/8)
• Integration testing confirms real contract functions work as expected

The fix is minimal and surgical, preserving all existing functionality while correctly handling the unit type case.

Fixes #1215.

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