test: add env-aware integration flow for sandbox and legacy PFX

[vitormattos]

Summary

• ignore local env/secret paths to keep credentials out of git
• add .env.example with sandbox integration variables
• add env loader helper for optional integration execution
• add integration test for sandbox mTLS HEAD check
• add integration test for real PFX signer path with runtime-compatibility skip behavior

Validation

• docker compose exec -T akaunting.php sh -lc "cd /var/www/html/packages/librecodeoop/nfse-php && composer test:unit"
• docker compose exec -T akaunting.php sh -lc "cd /var/www/html/packages/librecodeoop/nfse-php && composer test:integration"
• docker compose exec -T akaunting.php sh -lc "cd /var/www/html/packages/librecodeoop/nfse-php && composer cs:fix"
• docker compose exec -T akaunting.php sh -lc "cd /var/www/html/packages/librecodeoop/nfse-php && composer psalm"

Notes

• integration tests are optional and skip cleanly when env/certificate/runtime compatibility is unavailable
• unit tests remain mock-based and deterministic