Isolate app-server Unix sockets from filesystem-restricted commands - #45984

…45984)

## Why

Network access and Unix-socket allowlists must not let commands with filesystem restrictions reach the privileged app-server RPC transport.

## What changed

- Bind Unix control sockets in a fixed, user-owned directory with mode `0700`, independent of environment settings, and expose the advertised paths as symlinks. Preserve existing parent permissions, reject unsafe parents, and serialize socket setup and publication.
- Mask the socket directory in Linux bubblewrap sandboxes after each bind that exposes it. Reject host mount aliases and nested mounts that compromise isolation.
- Deny access to the directory and outbound connections to its sockets in macOS Seatbelt policies, including when network access or Unix-socket allowlists grant broader access.
- Require bubblewrap for filesystem-restricted Linux execution. Users with `features.use_legacy_landlock` enabled must disable it for these policies.

## Testing

Add regression coverage for direct and symlink socket access, hardlink attempts, Linux host-process links and bind-mount aliases, and continued use of unrelated and sandbox-local sockets. Add transport coverage for parent permissions, concurrent restart after a stale symlink, and cleanup that preserves a replacement at the advertised path.

GitOrigin-RevId: 53372c27eea278d964f2cf68aed24323ef3b7082