test: cover optional security fixture

[kriptoburak]

Summary

• Extends the existing security schemes fixture with an anonymous {} security alternative.
• Keeps the generated snapshot behavior pinned for specs that accept either public or API-key access.

Validation

• pnpm install --ignore-scripts --frozen-lockfile
• pnpm run build:debug
• pnpm exec ava __test__/generate.snapshot.spec.ts --match='generate preserves full success payload snapshot for security-schemes.openapi.yaml'
• git diff --check

Duplicate gate

• Checked memory, target tree, direct PR and issue lists, and search until GitHub Search returned a rate-limit response. No same-surface duplicate found.