test(snap): avoid pinning install fixtures to a published vite-plus version

[fengmk2]

PR #2017 temporarily pins VP_VERSION=0.2.1 in snap fixtures that run real package-manager installs.

This unblocks release CI because vp create / vp migrate now pin the exact CLI version, and release branches may reference a Vite+ version that is not published yet. Without the pin, those fixtures try to install vite-plus@<unpublished-version> / @voidzero-dev/vite-plus-core@<unpublished-version> and fail.

Context: #2017 (comment)

Follow-up: refactor these snap tests so they do not depend on a hard-coded published version. They should still pass on release branches before the release version is published, while keeping the snapshot output deterministic.

Acceptance criteria:

• Remove the hard-coded VP_VERSION=0.2.1 fixture pins added in test(snap): pin VP_VERSION to a published version in install fixtures #2017.
• Keep the affected install fixtures passing with an unpublished current Vite+ version.