test: cover clerk plugin auth and webhooks

[xingxi0614-cpu]

/claim #71

Summary:

• Added focused Vitest coverage for ClerkPlugin setup, webhook handling, and session authentication.
• Covered required signing-secret validation, middleware table setup, and the verifySessions: false setup path.
• Covered missing Svix headers, mismatched Clerk instance IDs, user upsert events, and revoked-session deletion events.
• Covered session auth disabled, cookie JWT verification, permitted-origin enforcement, and session lookup behavior.

Verification / demo:

• pnpm vitest --run plugins/clerk/index.test.ts -> 1 file passed, 10 tests passed
• pnpm exec prettier --check plugins/clerk/index.test.ts -> all matched files use Prettier code style
• git diff --check -> no whitespace errors

No production behavior was changed; this is intentionally scoped to plugins/clerk/index.test.ts and does not overlap with the active import/export/LiteREST/handler/worker/DO/operation/cache/RLS/allowlist/sql-macros/stats/cron/resend/studio/stripe/websocket coverage PRs.

AI-assisted with Codex; I reviewed the diff and kept the PR scoped to this Clerk plugin coverage gap.