fix: ignore "directory" blobs in the Azure backend when walking.

[peterhuene]

This PR fixes including "directory" blobs as output from the walk method, which should only be (conceptually) "files".

Azure Blob Storage has a feature for creating empty blobs that act as "directories" in a hierarchical namespace; the directory objects hold metadata such as Access Control Lists (ACLs).

By including the directory as output from the walk function, callers might attempt to download the directory as a file and then create it as a directory when also downloading the directory's contents, which would result in a conflicting file system error.

Additionally included a check for such conflicts so that walking returns an error.

Replaced localstack with floci for testing.

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[claymcleod]

One path-safety issue could not be attached inline: in src/transfer.rs:851, walked object names are joined into local destination paths without rejecting .. or other non-normal components, so a crafted object name can escape the requested download directory. That looks pre-existing, but the new walk-conflict validation is a natural place to cover it.