test: add route tests for certificates verify endpoint

[hussnainahmedd]

Summary

Adds the missing colocated test for the certificate verification route handler (src/app/api/certificates/verify/[id]/route.ts), following the same pattern as the merged email-verification route tests.

New file: src/app/api/certificates/verify/[id]/__tests__/route.test.ts

What the tests cover

• 200 — valid certificate: asserts the validated verification payload is returned, the Cache-Control: public, max-age=3600 header is set, and verifyCertificate is called with the route param id.
• 404 — certificate not found / revoked / invalid (verifyCertificate returns null): asserts valid: false and the expected error message.
• 500 — service payload fails CertificateVerificationSchema validation: asserts the "Verification response validation failed" error.
• 500 — verifyCertificate throws: asserts the "Failed to verify certificate" error.

The verifyCertificate service is mocked (vi.mock); the handler is driven with a plain Request plus a literal { params: { id } } context, matching the existing route-test style.

Testing notes

• pnpm vitest run src/app/api/certificates/verify/[id]/__tests__/route.test.ts — 4/4 tests pass.
• pnpm run type-check — no new errors (the only 2 errors are pre-existing in unrelated files SidebarNavigation.tsx / AccessibleModerationTools.tsx).
• ESLint ignores __tests__ files per repo config.

Closes #1416

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