[BugFix] Preserve Required flag for duplicate interpolation variables

[Cursx]

Summary

Fixes docker/compose#13718 — template.ExtractVariables now correctly aggregates Required=true constraints when an interpolation variable is referenced multiple times.

When a Compose configuration utilizes the same variable name using both required and optional notations (e.g., ${PIHOLE_DOMAIN:?} followed by ${PIHOLE_DOMAIN}), the parser previously overwrote the element completely. This caused the variable mapping to silently drop the stringency requirement (Required: false), leading docker compose config --variables to misreport requirement constraints. This patch introduces a mergeVariable function implementing a logical OR for the Required flag and merges DefaultValue / PresenceValue attributes safely.

Before

services:
  pihole:
    image: pihole/pihole:2026.02.0
    labels:
      - "traefik.http.routers.pihole.rule=Host(`${PIHOLE_DOMAIN:?}`)"
      - "traefik.http.middlewares.pihole-redirect.redirectregex.regex=^(https://${PIHOLE_DOMAIN})/?$"

Running docker compose config --variables --format json mistakenly prints:

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

After

With mergeVariable correctly aggregating constraints, running docker compose config --variables --format json accurately respects the strictest condition anywhere in the file:

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

Users are now correctly warned about missing required variables in pre-flight Compose validations, regardless of YAML declaration order.

Test results (18 passed)

=== RUN   TestExtractVariables
=== RUN   TestExtractVariables/duplicate-variable-required-preserved
=== RUN   TestExtractVariables/duplicate-variable-value-preserved
--- PASS: TestExtractVariables (0.00s)
    --- PASS: TestExtractVariables/empty (0.00s)
    --- PASS: TestExtractVariables/no-variables (0.00s)
    --- PASS: TestExtractVariables/variable-without-curly-braces (0.00s)
    --- PASS: TestExtractVariables/variable-without-curly-braces-and-with-number-suffix (0.00s)
    --- PASS: TestExtractVariables/variable (0.00s)
    ...
    --- PASS: TestExtractVariables/duplicate-variable-required-preserved (0.00s)
    --- PASS: TestExtractVariables/duplicate-variable-value-preserved (0.00s)
PASS
ok  	github.com/compose-spec/compose-go/v2/template	2.225s

Test plan

• 5 new tests: duplicate-variable-required-preserved, duplicate-variable-value-preserved, duplicate-variable-in-map, duplicate-variable-in-string, and duplicate-variable-conflict-default-required yielding combined Required: true behavior and cascading default values without data loss across all AST node types.
• All 16 existing ExtractVariable tests pass (no regression)