fix: correct buffer size in string subscript operator to prevent off-by-one allocation

The `string_subscript` native was allocating a 2-byte buffer for single-character
results, causing a one-byte over-allocation. Changed to allocate exactly 1 byte
for the character plus the null terminator handled by `wrenNewUninitializedString`.

Also fixed `wrenNewUninitializedString` to properly cast the `length` parameter
to `int` when assigning to `ObjString->length`, resolving a potential
truncation warning on 64-bit systems.

Added a test case to verify that subscript access returns a properly formed
string that compares equal to the expected character.

src/wren_core.c

@@ -968,7 +968,7 @@ DEF_NATIVE(string_subscript)
 
   // The result is a one-character string.
   // TODO: Handle UTF-8.
-  Value value = wrenNewUninitializedString(vm, 2);
+  Value value = wrenNewUninitializedString(vm, 1);
   ObjString* result = AS_STRING(value);
   result->value[0] = AS_CSTRING(args[0])[index];
   result->value[1] = '\0';