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/environment_context.rs

@@ -70,8 +70,8 @@ impl EnvironmentContext {
     ) -> Self {
         let before_network = Self::network_from_turn_context_item(before);
         let after_network = Self::network_from_turn_context(after);
-        let cwd = if before.cwd != after.cwd {
-            Some(after.cwd.clone())
+        let cwd = if before.cwd.as_path() != after.cwd.as_path() {
+            Some(after.cwd.to_path_buf())
         } else {
             None
         };
@@ -94,7 +94,7 @@ impl EnvironmentContext {
 
     pub fn from_turn_context(turn_context: &TurnContext, shell: &Shell) -> Self {
         Self::new(
-            Some(turn_context.cwd.clone()),
+            Some(turn_context.cwd.to_path_buf()),
             shell.clone(),
             turn_context.current_date.clone(),
             turn_context.timezone.clone(),