Add integration tests for the marketplace listing detail and purchase API routes

[1nonlypiece]

📌 Description

The marketplace API has several routes — listings/[id]/preflight/route.ts,
listings/[id]/purchase/route.ts, featured/route.ts, and stats/route.ts.
There are tests for listings/route.ts and listings/[id]/route.ts
(route.test.ts) and tests/api/marketplace-purchase.test.ts, but preflight,
featured, and stats lack dedicated coverage.

This issue closes the marketplace API integration-test gaps.

Goal: the preflight, featured, and stats marketplace routes are covered for
success, validation, and error responses.

🎯 Requirements and Context

• Add integration tests for listings/[id]/preflight (eligible vs ineligible
  buyer, sold-out listing).
• Add tests for marketplace/featured and marketplace/stats response shapes.
• Reuse tests/api/helpers.ts request utilities.
• Assert ETag/caching behavior where these routes use it
  (marketplace-cache-invalidation.test.ts patterns).

🛠️ Suggested Execution

1. Create a branch

git checkout -b feature/marketplace-api-tests

2. Implement changes

• Add tests/api/marketplace-preflight.test.ts,
  tests/api/marketplace-featured.test.ts, and
  tests/api/marketplace-stats.test.ts.

3. Test and commit

• Run npm test.
• Edge cases: missing listing id, ineligible buyer, empty featured set, stats
  with zero listings.

Example commit message

test: cover marketplace preflight, featured, and stats routes

✅ Guidelines

• Minimum 95% test coverage on the targeted routes.
• Clear, descriptive test names.
• Timeframe: 96 hours.

🏷️ Labels

type-testing · area-frontend · MAYBE REWARDED · GRANTFOX OSS · OFFICIAL CAMPAIGN

💬 Community & Support

• Join the CommitLabs contributor Discord: https://discord.gg/WV7tdYkJk
• Introduce yourself before starting to avoid duplicate work.
• Maintainers triage actively and review fast.

[jakerated-r]

Opened PR #610 for this: #610

Validation:

• npm ci
• npm test -- --run tests/api/marketplace-preflight.test.ts tests/api/marketplace-featured.test.ts tests/api/marketplace-stats.test.ts tests/api/marketplace-purchase.test.ts -> 4 files / 23 tests passed
• git diff --cached --check

Note: the targeted run passes and reports an existing Vite warning for duplicate marketplaceStats in src/lib/backend/cache/index.ts; this PR does not touch that module.