cgen: fix option sumtype if expr struct init

[medvednikov]

Summary

• Preserve the previous struct-init cgen context when emitting nested struct init expressions.
• Add a regression test for an option-of-sumtype field initialized via an if expression.

Fixes #27440

Testing

• ./vnew -silent test vlib/v/tests/options/option_sumtype_if_expr_struct_field_test.v (passed before the request to skip further tests)
• ./vnew -silent vlib/v/compiler_errors_test.v (passed before the request to skip further tests)
• Not completed: ./vnew -silent vlib/v/gen/c/coutput_test.v (stopped after the request to stop testing)