feat: Env helper — read process env, not just $_ENV / dotenv

[PrestaEdit]

Why

Historically the lib reads env vars via $_ENV['PRESTAFLOW_...']. PHP populates $_ENV from the process environment only if variables_order includes E — which is often absent in CI (e.g. GitHub Actions' shivammathur/setup-php). The lib relied on phpdotenv to load .env / .env.local files into $_ENV, but plain env vars set by the shell were silently ignored.

Concrete symptom (repro'd on prestaflow-prestashop#2): with env: PRESTAFLOW_PS_VERSION: '9.0.0' set in the workflow, the lib still resolved to v8 defaults, then crashed with Class \"...\\Pages\\v8\\Front\\GuestTracking\\Page\" not found because the version was invisible.

What

• src/Utils/Env.php — new helper.
  • Env::get($key, $default = null): mixed — reads $_ENV first (preserving dotenv values and the normalized values TestsSuite writes at load time), falls back to getenv(), then to the default. Empty string in $_ENV still wins over the default (preserves the previous ?? $default semantics).
  • Env::has($key): bool — true if either source has the key.
• Refactor sites (behaviour preserved everywhere):
  • TestsSuite.php — ~27 read sites; the 4 normalization blocks (DEBUG, HEADLESS, PREFIX_LOCALE, VERBOSE) now use Env::has()/Env::get() but keep writing the normalized value back to $_ENV so downstream direct reads (used inside the same big loadGlobals() block) see the bool. Deletes the stale variables_order comment near $envWidth/$envHeight.
  • Version.php, Screenshots.php, ExecuteSuite.php — 1 site each.
• 7 unit tests on the helper (Env::get order, Env::has, empty string handling, absent everywhere → default).

Impact for the GH Action

With this in prestaflow/php-library, PrestaFlow/github-action#7's .env.local workaround becomes unnecessary — consumers can just set env: in the workflow, or the action can keep exporting to the process env only. The workaround can stay as a safety net for older lib versions.

Test plan

• php -l clean on all 5 modified files + Env.php.
• composer test-unit: 173 tests, 388 assertions, all pass (was 166/380 before adding the 7 EnvTest cases).
• After merge + tagging, re-run github-action-smoke and prestaflow-prestashop#2 to confirm the class-not-found error is gone.

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