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/agent/role.rs

@@ -252,7 +252,7 @@ mod reload {
 
     fn reload_overrides(config: &Config, preserve_current_provider: bool) -> ConfigOverrides {
         ConfigOverrides {
-            cwd: Some(config.cwd.clone()),
+            cwd: Some(config.cwd.to_path_buf()),
             model_provider: preserve_current_provider.then(|| config.model_provider_id.clone()),
             codex_linux_sandbox_exe: config.codex_linux_sandbox_exe.clone(),
             main_execve_wrapper_exe: config.main_execve_wrapper_exe.clone(),