fix: strip export-from server data exports in client transform

[JaredStowell]

Motivation

• The client-side transform stripServerExports skipped ExportNamedDeclaration nodes that have a source (patterns like export { getServerSideProps } from './server'), leaving server-only data-fetching exports in client bundles and potentially exposing server-only code.

Description

• Update stripServerExports in packages/vinext/src/index.ts to process ExportNamedDeclaration specifiers regardless of node.source so export ... from '...' patterns are handled.
• Preserve non-server re-exported specifiers (including their from '...' source) while replacing server-only specifiers with stubbed export const <name> = undefined; lines for mixed re-exports.
• Add regression tests in tests/build-optimization.test.ts covering pure export-from server exports and mixed specifier cases to prevent regressions.

Testing

• Ran the focused test: pnpm vitest run tests/build-optimization.test.ts -t "handles export-from", and the targeted tests passed.
• Added tests are part of tests/build-optimization.test.ts and the new cases pass under the targeted Vitest run.

---

Codex Task