fix(sandbox): stop Docker recreating deleted files in the project

[radim10]

Summary

Fixes deleted files reappearing in the project after starting a new Docker sandbox session.

Problem

append_filesystem_mounts added a bind mount for every denied-read and denied-write path inside the project, whether or not the path exists on the host. When a mount target is missing inside the bind-mounted project folder, Docker creates a placeholder at that path on the host: an empty file for denied-read file paths, an empty directory for denied-write paths.

So when a user deleted one of these files (manually or via git) and started a new sandboxed session, the file came back every time.

Fix

Skip the mount for a denied path that doesn't exist on the host (src/handlers/run/docker_sandbox.rs). There's nothing to hide or protect, and it's the same kind of check already applied to paths outside the project folder.

Trade-offs

Paths that exist when the session starts behave exactly as before. For paths that don't exist at startup:

• Denied-write: the agent can now create the path and write into it. Before, it was blocked, at the cost of leaving the placeholder behind.
• Denied-read: a file created on the host during a session (e.g. a new .env) is visible to the agent. Before, the placeholder mount hid it for the whole session.

Possible follow-up: mount missing paths as empty in-memory mounts inside the container and delete any placeholders the sandbox itself created when the session ends. That would keep strict protection without leaving files behind.

Testing

• Added docker_run_command_skips_mounts_for_nonexistent_denied_paths
• All 52 docker sandbox tests pass (cargo test docker_sandbox)