fix: Fix tilde expansion to avoid absolute-path escape (#9621)

### Motivation
- Prevent inputs like `~//` or `~///etc` from expanding to arbitrary
absolute paths (e.g. `/`) because `Path::join` discards the left side
when the right side is absolute, which could allow config values to
escape `HOME` and broaden writable roots.

### Description
- In `codex-rs/utils/absolute-path/src/lib.rs` update
`maybe_expand_home_directory` to trim leading separators from the suffix
and return `home` when the remainder is empty so tilde expansion stays
rooted under `HOME`.
- Add a non-Windows unit test
`home_directory_double_slash_on_non_windows_is_expanded_in_deserialization`
that validates `"~//code"` expands to `home.join("code")`.

### Testing
- Ran `just fmt` successfully.
- Ran `just fix -p codex-utils-absolute-path` (Clippy autofix)
successfully.
- Ran `cargo test -p codex-utils-absolute-path` and all tests passed.

------
[Codex
Task](https://chatgpt.com/codex/tasks/task_i_697007481cac832dbeb1ee144d1e4cbe)

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

@@ -33,6 +33,10 @@ impl AbsolutePathBuf {
                 return home;
             }
             if let Some(rest) = path_str.strip_prefix("~/") {
+                let rest = rest.trim_start_matches('/');
+                if rest.is_empty() {
+                    return home;
+                }
                 return home.join(rest);
             }
         }
@@ -253,6 +257,20 @@ mod tests {
         assert_eq!(abs_path_buf.as_path(), home.join("code").as_path());
     }
 
+    #[cfg(not(target_os = "windows"))]
+    #[test]
+    fn home_directory_double_slash_on_non_windows_is_expanded_in_deserialization() {
+        let Some(home) = home_dir() else {
+            return;
+        };
+        let temp_dir = tempdir().expect("base dir");
+        let abs_path_buf = {
+            let _guard = AbsolutePathBufGuard::new(temp_dir.path());
+            serde_json::from_str::<AbsolutePathBuf>("\"~//code\"").expect("failed to deserialize")
+        };
+        assert_eq!(abs_path_buf.as_path(), home.join("code").as_path());
+    }
+
     #[cfg(target_os = "windows")]
     #[test]
     fn home_directory_on_windows_is_not_expanded_in_deserialization() {