putBlobsIfNeeded reads each blob's hash and size once, up front, and
returns only plain HashedVirtualFileRef values, so serializing an ActionResult
(disk, in-memory, or remote store) performs no file I/O and nothing re-stats a
blob after its CAS entry is written: a file vanishing once stored no longer
prevents the write, and I/O errors on an output file surface upfront at blob
storage time rather than mid-serialization.
Fixes#9349
Co-authored-by: Claude Opus 4.8 (1M context) <[email protected]>
