sandboxing: preserve denied paths when widening permissions (#13451)

## Why

After the split-policy plumbing landed, additional-permissions widening
still rebuilt filesystem access through the legacy projection in a few
places.

That can erase explicit deny entries and make the runtime treat a policy
as fully writable even when it still has blocked subpaths, which in turn
can skip the platform sandbox when it is still needed.

## What changed

- preserved explicit deny entries when merging additional read and write
permissions into `FileSystemSandboxPolicy`
- switched platform-sandbox selection to rely on
`FileSystemSandboxPolicy::has_full_disk_write_access()` instead of ad
hoc root-write checks
- kept the widened policy path in `core/src/exec.rs` and
`core/src/sandboxing/mod.rs` aligned so denied subpaths survive both
policy merging and sandbox selection
- added regression coverage for root-write policies that still carry
carveouts

## Verification

- added regression coverage in `core/src/sandboxing/mod.rs` showing that
root write plus carveouts still requires the platform sandbox
- 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/13451).
* #13453
* #13452
* __->__ #13451
* #13449
* #13448
* #13445
* #13440
* #13439

---------

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

codex-rs/core/src/sandboxing/mod.rs

@@ -35,7 +35,6 @@ use codex_protocol::permissions::FileSystemPath;
 use codex_protocol::permissions::FileSystemSandboxEntry;
 use codex_protocol::permissions::FileSystemSandboxKind;
 use codex_protocol::permissions::FileSystemSandboxPolicy;
-use codex_protocol::permissions::FileSystemSpecialPath;
 use codex_protocol::permissions::NetworkSandboxPolicy;
 use codex_protocol::protocol::NetworkAccess;
 use codex_protocol::protocol::ReadOnlyAccess;
@@ -215,7 +214,6 @@ fn additional_permission_roots(
     )
 }
 
-#[cfg_attr(not(test), allow(dead_code))]
 fn merge_file_system_policy_with_additional_permissions(
     file_system_policy: &FileSystemSandboxPolicy,
     extra_reads: Vec<AbsolutePathBuf>,
@@ -369,14 +367,7 @@ pub(crate) fn should_require_platform_sandbox(
     }
 
     match file_system_policy.kind {
-        FileSystemSandboxKind::Restricted => !file_system_policy.entries.iter().any(|entry| {
-            entry.access == FileSystemAccessMode::Write
-                && matches!(
-                    &entry.path,
-                    FileSystemPath::Special { value }
-                        if matches!(value, FileSystemSpecialPath::Root)
-                )
-        }),
+        FileSystemSandboxKind::Restricted => !file_system_policy.has_full_disk_write_access(),
         FileSystemSandboxKind::Unrestricted | FileSystemSandboxKind::ExternalSandbox => false,
     }
 }