Avoid AbsolutePathBuf::parent() panic under EMFILE by skipping re-absolutization (#12647)

Fixes #12216

Fixes a panic in `AbsolutePathBuf::parent()` when the process hits file
descriptor exhaustion (`EMFILE` / "Too many open files").

### Root cause

`AbsolutePathBuf::parent()` was re-validating the parent path via
`from_absolute_path(...).expect(...)`.

`from_absolute_path()` calls `path_absolutize::absolutize()`, which can
depend on `std::env::current_dir()`. Under `EMFILE`, that can fail,
causing `parent()` to panic even though the parent of an absolute path
is already known.

### Change

- Stop re-absolutizing the result of `self.0.parent()`
- Construct `AbsolutePathBuf` directly from the known parent path
- Keep an invariant check with `debug_assert!(p.is_absolute())`

### Why this is safe

`self` is already an `AbsolutePathBuf`, so `self.0` is
absolute/normalized. The parent of an absolute path is expected to be
absolute, so re-running fallible normalization here is unnecessary and
can introduce unrelated panics.

codex-rs/utils/absolute-path/src/lib.rs

@@ -69,8 +69,11 @@ impl AbsolutePathBuf {
 
     pub fn parent(&self) -> Option<Self> {
         self.0.parent().map(|p| {
-            #[expect(clippy::expect_used)]
-            Self::from_absolute_path(p).expect("parent of AbsolutePathBuf must be absolute")
+            debug_assert!(
+                p.is_absolute(),
+                "parent of AbsolutePathBuf must be absolute"
+            );
+            Self(p.to_path_buf())
         })
     }