`ExtractVariables` does not report a variable used inside a `${VAR:?…}` message

[DRMacIver]

A variable that appears only inside the message of a required-variable expansion is left out of ExtractVariables: ${U:?${B}} and ${U?$B} report U alone, while ${U:-${B}} and ${U:+${B}} report both U and B, and Substitute expands B into the message of the same template:

package main

import (
	"fmt"
	"sort"

	"github.com/compose-spec/compose-go/v2/template"
)

func main() {
	for _, tmpl := range []string{"${U:-${B}}", "${U:+${B}}", "${U:?${B}}", "${U?$B}"} {
		vars := template.ExtractVariables(map[string]interface{}{"k": tmpl}, nil)
		names := make([]string, 0, len(vars))
		for name := range vars {
			names = append(names, name)
		}
		sort.Strings(names)
		fmt.Printf("%-12s %v\n", tmpl, names)
	}

	mapping := func(name string) (string, bool) {
		if name == "B" {
			return "value of B", true
		}
		return "", false
	}
	_, err := template.SubstituteWithOptions("${U:?${B}}", mapping, template.WithoutLogging)
	fmt.Println(err)
}

Output:

${U:-${B}}   [B U]
${U:+${B}}   [B U]
${U:?${B}}   [U]
${U?$B}      [U]
required variable U is missing a value: value of B

The specification lists ${VARIABLE?$FOO} among the nested interpolations, and substitution does read B when building the message, so I expected ExtractVariables to report B for the :? and ? forms as it does for :- and :+.

Tested on compose-go v2.15.0 and on current main (f18e211).

BTW, this was found by an automated program that writes property-based tests for various open source projects using hegel (but it has been reviewed by hand before reporting). We've also potentially found (but not yet hand validated) 2 other bugs in compose-go. You can see the tests at https://github.com/hegeldev/hegel-zoo/tree/main/targets/go/compose-go. Let us know if you would like us to file the other bugs found and/or contribute the tests. NB the tests are currently LLM generated and probably not yet suitable for inclusion as is, but we're happy to help get them into a better state if you want them.