Make required variable reporting deterministic

[ousamabenyounes]

What I did

Make Compose required-variable handling deterministic before project interpolation runs.

• preflight the non-interpolated model for required variables and report the first missing variable in sorted order
• use a Compose-side variable extractor that walks maps deterministically
• merge duplicate variable occurrences so a required use is not overwritten by a non-required use

Related issue

Fixes #13712
Fixes #13718

Test verification (RED -> GREEN)

RED for #13712 on origin/main with the issue repro run 20 times:

5: error while interpolating services.traefik.volumes.[].source: required variable TRAEFIK_ACME_PATH is missing a value
11: error while interpolating services.traefik.volumes.[].source: required variable TRAEFIK_ACME_PATH is missing a value
15: error while interpolating services.traefik.volumes.[].source: required variable TRAEFIK_ACME_PATH is missing a value

GREEN for #13712 on this branch with the same repro:

1: required variable TIMEZONE is missing a value
2: required variable TIMEZONE is missing a value
3: required variable TIMEZONE is missing a value
10: required variable TIMEZONE is missing a value

RED for #13718 on origin/main:

{
    "PIHOLE_DOMAIN": {
        "Name": "PIHOLE_DOMAIN",
        "DefaultValue": "",
        "PresenceValue": "",
        "Required": false
    }
}

GREEN for #13718 on this branch:

{
    "PIHOLE_DOMAIN": {
        "Name": "PIHOLE_DOMAIN",
        "DefaultValue": "",
        "PresenceValue": "",
        "Required": true
    }
}

Full local suite:

go test ./pkg/compose -count=1
ok  	github.com/docker/compose/v5/pkg/compose	0.527s

make test
DONE 511 tests, 2 skipped in 10.199s