tests: harden skip-unused json2 test against helper refactors

[enghitalo]

Background

On #27535, the chatgpt-codex-connector review flagged that vlib/v/tests/skip_unused/generic_fn_instantiation_pruning_test.v would start failing. This PR fixes the underlying brittleness so the test no longer hard-codes a json2 implementation detail.

The bug

test_skip_unused_keeps_json2_embedded_struct_decode_helpers (regression for #26928) asserts that -skip-unused keeps generic json2 helpers reachable only through comptime $for field codegen. One assertion hard-coded the generated symbol of a specific helper:

assert res.output.contains('x__json2__create_value_from_optional_T_time__Time')

create_value_from_optional is a json2 implementation detail. #27535 removes it (the option payload is now built inline with $zero(field.typ.payload_type)), so the symbol disappears and this assertion fails.

Reproduction

Compile the test program to C and grep the symbols (this is exactly what the test does):

// issue_26928.v
module main
import time
import x.json2
struct Meta { created_at ?time.Time }
struct Req  { Meta name string }
fn main() { _ := json2.decode[Req]('{"name":"x"}') or { panic(err) } }

v -w -o - issue_26928.v | grep create_value_from_optional

• on master: x__json2__create_value_from_optional_T_time__Time is present → test passes.
• on the x.json2: decode option fields via comptime $zero/$new type accessors #27535 branch: the symbol is gone → test_skip_unused_keeps_json2_embedded_struct_decode_helpers fails at that assertion.

The fix

Assert the symbol that actually decodes the ?time.Time payload of the embedded Meta — x__json2__Decoder_decode_value_T_time__Time — instead of a specific helper name. In this program time.Time only appears as that optional embedded field, so the instantiation is reachable only through comptime $for field codegen — exactly the skip-unused guarantee #26928 is about. Crucially, it is emitted whether or not the json2 helpers exist, so the test no longer breaks when they are refactored.

Testing

v -w test vlib/v/tests/skip_unused/generic_fn_instantiation_pruning_test.v → passes both:

• on master (json2 unchanged), and
• against the x.json2: decode option fields via comptime $zero/$new type accessors #27535 branch (helper removed).

This is independent of #27535 and green on master today; once it lands, #27535 just needs a rebase to pick it up.

🤖 Generated with Claude Code

[enghitalo]

Folding this into #27535 instead — the test fix now travels with the change that removes the helper. Closing in favor of that single PR.