fix: tighten up checks against writable folders for SandboxPolicy (#2338)

I was looking at the implementation of `Session::get_writable_roots()`,
which did not seem right, as it was a copy of writable roots, which is
not guaranteed to be in sync with the `sandbox_policy` field.

I looked at who was calling `get_writable_roots()` and its only call
site was `apply_patch()` in `codex-rs/core/src/apply_patch.rs`, which
took the roots and forwarded them to `assess_patch_safety()` in
`safety.rs`. I updated `assess_patch_safety()` to take `sandbox_policy:
&SandboxPolicy` instead of `writable_roots: &[PathBuf]` (and replaced
`Session::get_writable_roots()` with `Session::get_sandbox_policy()`).

Within `safety.rs`, it was fairly easy to update
`is_write_patch_constrained_to_writable_paths()` to work with
`SandboxPolicy`, and in particular, it is far more accurate because, for
better or worse, `SandboxPolicy::get_writable_roots_with_cwd()` _returns
an empty vec_ for `SandboxPolicy::DangerFullAccess`, suggesting that
_nothing_ is writable when in reality _everything_ is writable. With
this PR, `is_write_patch_constrained_to_writable_paths()` now does the
right thing for each variant of `SandboxPolicy`.

I thought this would be the end of the story, but it turned out that
`test_writable_roots_constraint()` in `safety.rs` needed to be updated,
as well. In particular, the test was writing to
`std::env::current_dir()` instead of a `TempDir`, which I suspect was a
holdover from earlier when `SandboxPolicy::WorkspaceWrite` would always
make `TMPDIR` writable on macOS, which made it hard to write tests to
verify `SandboxPolicy` in `TMPDIR`. Fortunately, we now have
`exclude_tmpdir_env_var` as an option on
`SandboxPolicy::WorkspaceWrite`, so I was able to update the test to
preserve the existing behavior, but to no longer write to
`std::env::current_dir()`.







---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/2338).
* #2345
* #2329
* #2343
* #2340
* __->__ #2338

codex-rs/core/src/protocol.rs

@@ -156,10 +156,31 @@ pub enum SandboxPolicy {
 /// not modified by the agent.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct WritableRoot {
+    /// Absolute path, by construction.
     pub root: PathBuf,
+
+    /// Also absolute paths, by construction.
     pub read_only_subpaths: Vec<PathBuf>,
 }
 
+impl WritableRoot {
+    pub(crate) fn is_path_writable(&self, path: &Path) -> bool {
+        // Check if the path is under the root.
+        if !path.starts_with(&self.root) {
+            return false;
+        }
+
+        // Check if the path is under any of the read-only subpaths.
+        for subpath in &self.read_only_subpaths {
+            if path.starts_with(subpath) {
+                return false;
+            }
+        }
+
+        true
+    }
+}
+
 impl FromStr for SandboxPolicy {
     type Err = serde_json::Error;