fix(rsc): align export async function checks

[james-elicx]

Description

The wrap-export and proxy-export files were both checking async functions slightly differently. The proxy-export file's version was less permissive than the wrap-export one. This changes them to share the same logic (and adds class expressions as well at the same time).

I believe this was the cause of cloudflare/vinext#722 and should potentially fix it.

[hi-ogawa]

Thanks for the fix! I move the util and unify the two test cases.

[Copilot]

Pull request overview

This PR unifies how wrap-export and proxy-export validate exported async functions in the RSC transform pipeline, making the checks consistent and more permissive where appropriate (including handling class expressions), which should address the reported mismatch behavior.

Changes:

• Centralize rejectNonAsyncFunction validation into transforms/utils.ts and reuse it from both transforms.
• Align proxy-export validation behavior with wrap-export (and add rejection for class expressions when enabled).
• Move/expand the validation test coverage into a shared utils.test.ts that exercises both transforms.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/plugin-rsc/src/transforms/wrap-export.ts	Removes local validation helper and delegates to shared validateNonAsyncFunction.
packages/plugin-rsc/src/transforms/wrap-export.test.ts	Removes the now-redundant per-transform validation test.
packages/plugin-rsc/src/transforms/utils.ts	Adds shared validateNonAsyncFunction implementation (including class expressions).
packages/plugin-rsc/src/transforms/utils.test.ts	Adds consolidated tests verifying accepted/rejected patterns for both transforms.
packages/plugin-rsc/src/transforms/proxy-export.ts	Replaces stricter bespoke validation with shared logic; validates variable initializers individually.

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