Use AbsolutePathBuf for cwd state (#15710)

Migrate `cwd` and related session/config state to `AbsolutePathBuf` so
downstream consumers consistently see absolute working directories.

Add test-only `.abs()` helpers for `Path`, `PathBuf`, and `TempDir`, and
update branch-local tests to use them instead of
`AbsolutePathBuf::try_from(...)`.

For the remaining TUI/app-server snapshot coverage that renders absolute
cwd values, keep the snapshots unchanged and skip the Windows-only cases
where the platform-specific absolute path layout differs.

codex-rs/core/src/project_doc.rs

@@ -184,7 +184,7 @@ pub async fn read_project_docs(config: &Config) -> std::io::Result<Option<String
 /// directory (inclusive). Symlinks are allowed. When `project_doc_max_bytes`
 /// is zero, returns an empty list.
 pub fn discover_project_doc_paths(config: &Config) -> std::io::Result<Vec<PathBuf>> {
-    let mut dir = config.cwd.clone();
+    let mut dir = config.cwd.to_path_buf();
     if let Ok(canon) = normalize_path(&dir) {
         dir = canon;
     }