sync: create missing parent hierarchy for remote beta roots

[Doow501]

What does this pull request do and why is it needed?

This pull request allows writable remote beta endpoints to create the missing
parent directory hierarchy for a synchronization root when the root itself does
not yet exist.

Today, synchronization can proceed if the root is missing but its parent
directory already exists. However, if the parent hierarchy is missing,
transition fails when attempting to walk to the synchronization root parent.

This change keeps the behavior narrow:

• it only applies to remote beta endpoints
• it only runs when the synchronization root itself does not exist
• it only creates the missing parent directory hierarchy
• it does not eagerly create the root leaf itself
• existing behavior is preserved when the root already exists
• clear errors are preserved if parent directory creation fails

This is needed to make first-time remote synchronization setup work more
smoothly when the target directory hierarchy has not yet been created on the
remote side.

Which issue(s) does this pull request address (if any)?

Closes #553

Any other notes for the review process?

This PR also adds tests covering:

• creation of missing parent directories without eagerly creating the root
• no-op behavior when the root already exists