refactor: centralize package metadata handling in build script ♻️

**♻️ Refactors**
- Add withPackageMetadata helper to manage preview and conditionally strip enabledApiProposals, run an action, and reliably restore package.json afterwards.
- Replace the previous ad-hoc strip/restore logic with withPackageMetadata calls in the marketplace, package, and package:dev build flows to consolidate behavior.

**🧹 Chores**
- Remove legacy stripProposals and restoreProposals functions from scripts/build.mjs.
- Move enabledApiProposals entry in package.json to the end of the file.