Issue #3593541: pdv_file materializes a vault-picked file into a local upload by default

What

Choosing a file from the vault is now materialized into a normal local managed file on the submission by default - handled exactly like a manual upload (real file entity, local download, present in exports and emails). Only the vault-pick path changes; manual uploads and the optional save-to-vault are untouched.

A new off-by-default element setting, Link to the vault file, keeps the previous reference-only behavior (no local copy; bytes read from the vault on download).

Real filename + MIME (read contract)

The materialized file is named and typed from the vault, not guessed. The file-read contract now returns the bytes plus the stored MIME and a server-generated filename - the item's kind plus the MIME's extension, e.g. id_card.pdf - never the owner's original filename:

• VaultClientInterface::readFile() returns a VaultFile value object (bytes, mime, filename).
• ConsumerApi::readFile() generates the name + MIME server-side.
• The cross-site /raw endpoint sends them as Content-Type + Content-Disposition; RemoteVaultClient reads them back.
• The webform download controller serves the real MIME (attachment disposition) instead of application/octet-stream.

Implementation

Materialization runs in the handler's preSave, so the pdv_file composite's own postSave makes the file permanent and tracks its usage - the same path a manual upload takes.

Tests

• materialize-by-default; link mode keeps the reference;
• permanent + usage-tracked after a full save();
• read-failure fallback (keeps the reference);
• MIME + filename on both the same-site client and the cross-site /raw endpoint.