[codex] Make AbsolutePathBuf joins infallible (#16981)

Having to check for errors every time join is called is painful and
unnecessary.

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

@@ -1449,7 +1449,7 @@ impl Config {
         let mut additional_writable_roots: Vec<AbsolutePathBuf> = additional_writable_roots
             .into_iter()
             .map(|path| AbsolutePathBuf::resolve_path_against_base(path, resolved_cwd.as_path()))
-            .collect::<Result<Vec<_>, _>>()?;
+            .collect();
         let active_project = cfg
             .get_active_project(resolved_cwd.as_path())
             .unwrap_or(ProjectConfig { trust_level: None });