test: cover stripe subscription plugin

[Treasure520520]

/claim #71

Summary

• Adds focused Vitest coverage for the Stripe subscription plugin without touching production behavior.
• Covers constructor validation, subscription table initialization, customer lookup/update vs creation, product-to-price subscription creation, missing payment-method rejection, unsubscribe success/not-found behavior, and subscription deletion / checkout completion webhooks.
• Mocks Stripe API calls with vi.stubGlobal('fetch', ...), so tests do not call the real Stripe API or require Stripe credentials.

Verification

• COREPACK_ENABLE_PROJECT_SPEC=0 corepack pnpm exec prettier --check plugins/stripe/index.test.ts
• COREPACK_ENABLE_PROJECT_SPEC=0 corepack pnpm vitest plugins/stripe/index.test.ts --run -> 11 tests passed
• git diff --check
• COREPACK_ENABLE_PROJECT_SPEC=0 corepack pnpm vitest --run -> new Stripe tests passed; existing unrelated src/rls/index.test.ts baseline still has 4 failures around SELECT/JOIN/subquery policy injection.
• COREPACK_ENABLE_PROJECT_SPEC=0 corepack pnpm exec tsc --noEmit --pretty false -> existing repo type errors remain in CDC/cache/DO/operation files; no errors from plugins/stripe/index.test.ts.

Notes

This is intentionally a non-overlapping plugin coverage slice, separate from the active/rewarded import/export/LiteREST/handler/worker/DO/operation/cache/RLS/allowlist/sql-macros/stats/cron/resend/studio attempts.