sandboxing: plumb split sandbox policies through runtime (#13439)

## Why

`#13434` introduces split `FileSystemSandboxPolicy` and
`NetworkSandboxPolicy`, but the runtime still made most execution-time
sandbox decisions from the legacy `SandboxPolicy` projection.

That projection loses information about combinations like unrestricted
filesystem access with restricted network access. In practice, that
means the runtime can choose the wrong platform sandbox behavior or set
the wrong network-restriction environment for a command even when config
has already separated those concerns.

This PR carries the split policies through the runtime so sandbox
selection, process spawning, and exec handling can consult the policy
that actually matters.

## What changed

- threaded `FileSystemSandboxPolicy` and `NetworkSandboxPolicy` through
`TurnContext`, `ExecRequest`, sandbox attempts, shell escalation state,
unified exec, and app-server exec overrides
- updated sandbox selection in `core/src/sandboxing/mod.rs` and
`core/src/exec.rs` to key off `FileSystemSandboxPolicy.kind` plus
`NetworkSandboxPolicy`, rather than inferring behavior only from the
legacy `SandboxPolicy`
- updated process spawning in `core/src/spawn.rs` and the platform
wrappers to use `NetworkSandboxPolicy` when deciding whether to set
`CODEX_SANDBOX_NETWORK_DISABLED`
- kept additional-permissions handling and legacy `ExternalSandbox`
compatibility projections aligned with the split policies, including
explicit user-shell execution and Windows restricted-token routing
- updated callers across `core`, `app-server`, and `linux-sandbox` to
pass the split policies explicitly

## Verification

- added regression coverage in `core/tests/suite/user_shell_cmd.rs` to
verify `RunUserShellCommand` does not inherit
`CODEX_SANDBOX_NETWORK_DISABLED` from the active turn
- added coverage in `core/src/exec.rs` for Windows restricted-token
sandbox selection when the legacy projection is `ExternalSandbox`
- updated Linux sandbox coverage in
`linux-sandbox/tests/suite/landlock.rs` to exercise the split-policy
exec path
- verified the current PR state with `just clippy`




---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/13439).
* #13453
* #13452
* #13451
* #13449
* #13448
* #13445
* #13440
* __->__ #13439

---------

Co-authored-by: viyatb-oai <viyatb@openai.com>

codex-rs/linux-sandbox/tests/suite/landlock.rs

@@ -9,6 +9,8 @@ use codex_core::exec::process_exec_tool_call;
 use codex_core::exec_env::create_env;
 use codex_core::sandboxing::SandboxPermissions;
 use codex_protocol::config_types::WindowsSandboxLevel;
+use codex_protocol::permissions::FileSystemSandboxPolicy;
+use codex_protocol::permissions::NetworkSandboxPolicy;
 use codex_protocol::protocol::SandboxPolicy;
 use codex_utils_absolute_path::AbsolutePathBuf;
 use pretty_assertions::assert_eq;
@@ -102,6 +104,8 @@ async fn run_cmd_result_with_writable_roots(
     process_exec_tool_call(
         params,
         &sandbox_policy,
+        &FileSystemSandboxPolicy::from(&sandbox_policy),
+        NetworkSandboxPolicy::from(&sandbox_policy),
         sandbox_cwd.as_path(),
         &codex_linux_sandbox_exe,
         use_bwrap_sandbox,
@@ -333,6 +337,8 @@ async fn assert_network_blocked(cmd: &[&str]) {
     let result = process_exec_tool_call(
         params,
         &sandbox_policy,
+        &FileSystemSandboxPolicy::from(&sandbox_policy),
+        NetworkSandboxPolicy::from(&sandbox_policy),
         sandbox_cwd.as_path(),
         &codex_linux_sandbox_exe,
         false,