Add agent-server endpoint for archiving workspace files

[neubig]

Goal

Expose a safe agent-server endpoint that returns an archive of the active workspace files so callers can persist the agent's work before a runtime is deleted. In some cases we want to have a long-lived archive of the agent's work to prevent it from being irrecoverable.

Scope

Add or harden an endpoint in openhands-agent-server for downloading a directory archive.

Relevant code path:

• openhands-agent-server/openhands/agent_server/file_router.py
  • Existing /file/upload and /file/download endpoints.
  • Existing _create_zip_from_directory() helper used for trajectory export.

Proposed implementation

1. Add an endpoint such as GET /file/archive?path=/workspace/project.
2. Return a streamed .zip or .tar.gz archive of the requested directory.
3. Validate that the requested path is absolute, exists, and is a directory.
4. Prevent path traversal and avoid following symlinks outside the requested root.
5. Prefer streaming or temp-file cleanup via BackgroundTask so large workspaces do not remain on disk after response completion.
6. Add a manifest entry or deterministic archive metadata where practical.

Acceptance criteria

• A caller can request an archive of /workspace/project and receive a valid archive.
• Requests for missing paths, files instead of directories, or unsafe paths fail with clear 4xx responses.
• Symlinks do not allow files outside the requested root to be included.
• Tests cover nested files, empty directories, unsafe symlinks, missing paths, and cleanup of temporary archive files.
• The endpoint is documented in the agent-server OpenAPI output.