Track vary params during runtime prefetches (#89297)
**Previous:**
1. https://github.com/vercel/next.js/pull/91487
2. https://github.com/vercel/next.js/pull/91488
**Current:**
3. https://github.com/vercel/next.js/pull/89297
---
Most of the vary params infrastructure was already implemented in
previous PRs for static prerenders. This wires up the remaining pieces
for runtime prefetches — creating the accumulator, setting it on the
prerender store, and resolving it before abort — and adds additional
test cases covering empty/full vary sets, searchParams, metadata, and
per-segment layout/page splits with runtime prefetching.
