test: check ReadList return value #338

ReadList was originally declared void and did not return the result of
read_dest.update(), silently discarding the return value. When callers
pass a ReadDestTemp or other ReadDest whose update() returns a
constructed value, that value was lost.

Add a test to demonstrate the bug: extend FooCustom with a v3
:List(Int32) / std::vector<int> field and use the assignment pattern
  value.v3 = ReadField(..., ReadDestTemp<std::vector<int>>())
inside CustomReadField. This fails to compile if ReadList returns void
and succeeds with the fix.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>