refactor: split out validateStaticShell (#95106)
This is a pure refactor, split from #94595 to reduce the diff
- Move static shell validation code into a separate function,
`validateStaticShell`
- Rename `spawnStaticShellValidationInDev` to `runValidationInDev`. This
is outdated: it no longert spawns anything, and also does more than just
static shell validation
Also includes a drive-by cleanup in
`renderWithWarmCachesForValidationInDev` that was not worth a separate
PR : we can drop an unnecessary `accumulatedChunksPromise` wrapper,
because it's immediately awaited.
