test: add stats plugin coverage

[sureshchouksey8]

/claim #71

Purpose

Adds focused Vitest coverage for the stats plugin slice from #71.

Tasks

• Cover initialization and internal stats route registration.
• Cover middleware capture of request config and data source.
• Cover non-admin rejection before statistics are requested.
• Cover admin stats responses with registry plugin names.
• Cover the admin response when data source stats are unavailable.

Verify

• npx pnpm@9.15.4 vitest run plugins/stats/index.test.ts passes locally.
• npx pnpm@9.15.4 exec prettier --write plugins/stats/index.test.ts completed after formatting.
• git diff --check passes locally.