feat(esm): Add missing dependency to packages/testing (take 2)

[Tobbe]

unplugin-auto-import is used in vite plugins that we added in #355 and then again in #362

This PR is re-applying #360

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[greptile-apps]

Greptile Summary

This PR adds the missing unplugin-auto-import dependency to the @cedarjs/testing package. The dependency is required for the autoImportsPlugin function that was introduced in previous PRs (#355 and #362) which provides automatic imports for testing utilities like mockContext, mockHttpEvent, and mockSignedWebhook from @cedarjs/testing/api, as well as gql from graphql-tag and context from @cedarjs/context.

The change fixes a runtime dependency issue where the testing package's vitest configuration would fail because unplugin-auto-import/vite was being imported but the dependency wasn't declared in package.json. Additionally, the PR includes a minor path correction to the build script by adding a ./ prefix to tsconfig.cjs.json for consistency with other TypeScript config references in the same file.

This change ensures that consumers of the @cedarjs/testing package can successfully use the vitest integration features that were added in the recent ESM preparation work. The dependency is properly versioned at 19.3.0 and fits into the existing testing package architecture within the CedarJS monorepo.

Confidence score: 5/5

• This PR is safe to merge with minimal risk
• Score reflects a straightforward dependency fix with clear justification and proper versioning
• No files require special attention

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