Motivation

• A regression allowed policies with missing or empty run_as_user/run_as_group to skip privilege dropping and run sandboxed children as root, weakening isolation.
• The runtime must ensure the process identity is normalized before any privilege-drop or filesystem ownership operations so a missing identity cannot bypass security checks.

Description

• Change run_sandbox to retain a mutable policy returned from load_policy and pass a mutable reference into validate_sandbox_user so normalization happens at startup (crates/openshell-sandbox/src/lib.rs).
• Update validate_sandbox_user signature to accept &mut SandboxPolicy and fill missing or empty process.run_as_user and process.run_as_group with the literal string "sandbox" before verifying the account exists in the image (crates/openshell-sandbox/src/lib.rs).
• Add two regression unit tests that assert missing and empty process identity fields are populated to sandbox by the validation function (validate_sandbox_user_fills_missing_process_identity and validate_sandbox_user_fills_empty_process_identity in crates/openshell-sandbox/src/lib.rs).
• Keep existing behavior of failing fast if the sandbox account is absent, preserving the preexisting safety check while preventing noop privilege drops.

Testing

• Ran the unit tests in the sandbox crate targeting the new checks with cargo test -p openshell-sandbox and verified the new tests validate_sandbox_user_fills_missing_process_identity and validate_sandbox_user_fills_empty_process_identity passed.
• Ran cargo fmt --all --check which succeeded in this environment.
• Attempted mise run pre-commit but it could not complete in this environment due to external tool metadata resolution and task discovery failures (environment/network limitation), so pre-commit hooks were not validated here.

Codex Task