test: cover stats plugin

[Treasure520520]

/claim #71

Summary

• add focused Vitest coverage for the Stats plugin
• cover plugin metadata, non-admin rejection, admin stats responses, current plugin list inclusion, missing data source behavior, and per-request middleware refresh of config/dataSource
• use a Hono test app and mocked data source/registry methods so the route behavior is exercised without external services

Verification

• COREPACK_ENABLE_PROJECT_SPEC=0 corepack pnpm exec prettier --check plugins/stats/index.test.ts
• COREPACK_ENABLE_PROJECT_SPEC=0 corepack pnpm vitest plugins/stats/index.test.ts --run
• git diff --check
• COREPACK_ENABLE_PROJECT_SPEC=0 corepack pnpm exec tsc --noEmit --pretty false still reports existing repository errors in plugins/cdc, src/cache, src/do, and src/operation; no errors mention plugins/stats
• COREPACK_ENABLE_PROJECT_SPEC=0 corepack pnpm vitest --run still reports the existing 4 RLS failures in src/rls/index.test.ts; the new Stats test file passes in the full run

Notes

This is a non-overlapping coverage slice for #71, separate from the Stripe, WebSocket, Clerk, CDC, SQL macros, and Resend plugin PRs I submitted earlier.