Issue #3595220: Hide the pdv_file save-to-vault checkbox when the vault is locked

Fixes the bug reported in this issue.

A passphrase-locked vault correctly shows the unlock prompt and hides the record store-consent checkboxes, but the pdv_file "Save or update this file in my vault for reuse" checkbox stayed visible. The save controls are composite children built in the element #process pass (after the handler alterForm), so hiding them by key was a no-op. This clears #offer_vault_save on every pdv_file element on the locked and unreachable-vault paths, so the composite never emits the save controls.

Adds a kernel test (testLockedVaultHidesFileSaveOption) that fails without the fix.