[Bug]: SMB upload parent creation silently suppresses errors and may miss nested directories #184

Problem

On current DBackup dev (and v4.0.0), src/lib/adapters/storage/smb.ts has an ensureDir() helper that calls client.mkdir(path.posix.dirname(remotePath), "/") once and catches every error as though the directory already existed.

For nested backup destination paths, an SMB server may require each missing parent to be created separately. If creation fails due to access denial or a genuinely missing ancestor, the catch-all leaves the failure unexplained until a later sendFile() attempt. The adapter's existing session directory cache can also mark the attempted path as known before a usable directory has been proven.

Proposed behavior

• Create each absent parent directory in order below the configured SMB pathPrefix; treat the configured destination root as an existing boundary rather than automatically traversing/creating paths above it.
• Cache only paths that were successfully created or confirmed to already exist.
• Ignore only recognized benign already-exists responses (NT_STATUS_OBJECT_NAME_COLLISION and equivalent server messages); propagate unrelated mkdir errors with a safe path-specific message.
• Reject path traversal such as .. and absolute destination paths that escape the configured root, while preserving ordinary relative paths.
• Include tests for nested parent creation order, root confinement, idempotent already-existing directories and permission-denied failure.

The current downstream has several iteratively hardened regression tests for this behavior. This is a single generic SMB adapter correctness patch and requires neither a database migration nor environment-specific configuration.

The sibling issues #181 and #182 concern SMB listing completeness and long-filename enumeration respectively, and should remain separate from parent creation.