Server actions: `serverActions.bodySizeLimit` not enforced

[james-elicx]

This issue was created by an agent auditing CI failures from the Next.js Deploy Suite (vinext main vs Next.js v16.2.6, 2026-06-08).

Problem

serverActions.bodySizeLimit is not enforced: server-action requests that exceed the configured limit are not rejected. Fails for both plaintext and multipart fetch actions, with and without node-runtime middleware.

Affected Test Suites

• test/e2e/app-dir/actions/app-action-size-limit-invalid.test.ts (2 failures — plaintext, multipart; "should error for requests that exceed the size limit")
• test/e2e/app-dir/actions/app-action-size-limit-invalid-node-middleware.test.ts (2 failures — same, with node-runtime middleware)

Estimated impact

~4 e2e failures across 2 suites.

Notes

Not covered by the existing server-action issues #1340 / #1480.

Recommendation

1. Reproduce: set serverActions.bodySizeLimit and POST an oversized action body; assert the request errors instead of succeeding.
2. Enforce the configured limit in the action-request handling path (both plaintext and multipart), including when a node-runtime middleware is present.