feat: implement secure path validation for downloadManyFiles

[thiyaguk09]

• Adds protection against path traversal (../) using normalized path resolution.
• Prevents Windows-style drive letter injection while allowing GCS timestamps.
• Implements directory jail logic to ensure absolute-style paths are relative to destination.
• Preserves backward compatibility by returning an augmented DownloadResponse array.
• Automates recursive directory creation for validated nested files.
• Adds comprehensive 13-scenario test suite for edge-case parity.