test: add Stripe subscription plugin coverage

[RupteRyuxh]

Summary

• Add focused Vitest coverage for StripeSubscriptionPlugin initialization and helper behavior.
• Cover Stripe API request encoding/error propagation, customer lookup/create behavior, product-to-price subscription flow, payment-method guard, cancellation, and subscription persistence through the Starbase data source context.
• Test-only change; no production behavior changes.

/claim #71

Validation

• ./node_modules/.bin/vitest run plugins/stripe/index.test.ts -> 11 tests passed
• ./node_modules/.bin/prettier --check plugins/stripe/index.test.ts -> passed
• git diff --check -> passed

Non-overlap check

Before starting, I searched open PRs for stripe, StripeSubscriptionPlugin, and plugins/stripe and did not find an existing same-scope coverage PR.

[RupteRyuxh]

Local validation passes on my side:

• ./node_modules/.bin/vitest run plugins/stripe/index.test.ts -> 11 tests passed
• ./node_modules/.bin/prettier --check plugins/stripe/index.test.ts -> passed
• git diff --check -> passed

The GitHub Actions run appears to be waiting for maintainer approval because this PR comes from a fork.