test: cover printable typeof on sumtypes

[vincenzopalazzo]

Summary

• add regression coverage for using raw typeof(sumtype) as a printable string expression
• lock in the behavior reported in Printing typeof() fails to compile #26704 without changing current cgen logic

Root cause

The original issue was a codegen regression in older compiler builds when println(typeof(step)) was used on a sumtype. The current tree no longer reproduces that failure, but there was no targeted test covering that path.

Verification

• ./vnew -gc none -silent vlib/v/tests/typeof_test.v
• ./vnew -g -o /tmp/issue26704_pr.c /tmp/issue26704_pr.v
• ./vnew -gc none /tmp/issue26704_pr.v && /tmp/issue26704_pr

Fixes #26704