tests: assert the json2 payload decoder, not a helper name, in skip-unused test

`test_skip_unused_keeps_json2_embedded_struct_decode_helpers` hard-coded the
generated symbol `x__json2__create_value_from_optional_T_time__Time`. That helper
is a json2 implementation detail removed by this PR (the option payload is now
built inline with `$zero(field.typ.payload_type)`), after which the symbol no
longer exists and the test would fail.

Assert `x__json2__Decoder_decode_value_T_time__Time` instead -- the generic
instantiation that actually decodes the `?time.Time` payload of the embedded
`Meta` struct, reachable only through comptime `$for field` codegen. It is the
symbol skip-unused must keep (the point of issue #26928), is present whether or
not the json2 helpers exist, and so the test no longer breaks when they are
refactored.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: enghitalo

vlib/v/tests/skip_unused/generic_fn_instantiation_pruning_test.v

@@ -155,7 +155,11 @@ fn test_skip_unused_keeps_json2_embedded_struct_decode_helpers() {
 	}
 	assert res.output.contains('x__json2__decode_struct_key_T_main__Req')
 	assert res.output.contains('x__json2__check_required_struct_fields_T_main__Req')
-	assert res.output.contains('x__json2__create_value_from_optional_T_time__Time')
+	// the `?time.Time` payload of the embedded `Meta` is decoded through a generic
+	// instantiation reachable only via comptime `$for field` codegen; skip-unused
+	// must keep it. Assert the payload decoder itself rather than a specific json2
+	// helper name, so the test does not break when those helpers are refactored.
+	assert res.output.contains('x__json2__Decoder_decode_value_T_time__Time')
 }
 
 fn test_skip_unused_marks_dependencies_inside_generic_anon_fns() {