Harden managed proxy listener handoff (#40999)

## Why

Linux managed networking routed proxy connections through filesystem-backed Unix sockets, requiring an extra readable sandbox path and stale socket cleanup.

## What changed

- Transfer loopback TCP listeners from the sandbox namespace to host bridge processes over anonymous Unix socket pairs.
- Validate transferred descriptors, keep bootstrap channels private to each sandbox launch, and close them before running untrusted commands.
- Tie proxy bridge lifetime to the sandbox so cancellation closes active upstream connections.

## Testing

- Cover descriptor validation and interrupted handoffs.
- Verify concurrent sandboxes keep proxy endpoints isolated and do not inherit privileged sockets.
- Exercise managed networking end to end through `app-server`, confirming proxied access succeeds while direct host access remains blocked.

GitOrigin-RevId: 6561e09c83b927c604951bf2aa00ff532d179da5

codex-rs/linux-sandbox/Cargo.toml

@@ -27,6 +27,7 @@ codex-utils-absolute-path = { workspace = true }
 globset = { workspace = true }
 landlock = { workspace = true }
 libc = { workspace = true }
+rustix = { workspace = true }
 seccompiler = { workspace = true }
 serde = { workspace = true, features = ["derive"] }
 serde_json = { workspace = true }
@@ -37,6 +38,7 @@ url = { workspace = true }
 codex-core = { workspace = true }
 pretty_assertions = { workspace = true }
 tempfile = { workspace = true }
+test-case = "3.3.1"
 tokio = { workspace = true, features = [
     "io-std",
     "macros",