Restrict macOS scratch access to process sandboxes (#40961)

## Why

Filesystem helpers should not inherit access to system scratch directories beyond the paths granted by their filesystem policy, while ordinary processes still need scratch access for compatibility.

## What changed

- Move the `/tmp` and `/var/tmp` read/write grants, including their `/private` aliases, from the shared platform defaults into the process-only Seatbelt defaults.
- Keep process access to `/Applications` alongside the scratch-directory grants.

## Testing

Add a macOS Seatbelt test that verifies the generated policies and, when nested Seatbelt execution is available, confirms process scratch access, helper denial, and helper access to an approved project path.

GitOrigin-RevId: 26d83f09c131c1f1d000163b2f8929f601749719