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/cli/src/debug_sandbox.rs

@@ -260,7 +260,7 @@ async fn run_command_under_sandbox(
                 PathBuf::from("/usr/bin/sandbox-exec"),
                 args,
                 /*arg0*/ None,
-                cwd,
+                cwd.to_path_buf(),
                 network_policy,
                 env,
                 |env_map| {
@@ -293,7 +293,7 @@ async fn run_command_under_sandbox(
                 codex_linux_sandbox_exe,
                 args,
                 Some("codex-linux-sandbox"),
-                cwd,
+                cwd.to_path_buf(),
                 network_policy,
                 env,
                 |env_map| {