Require absolute cwd in thread settings (#26532)

## Why

Thread settings cwd overrides are expected to be resolved before they
enter core. Keeping this boundary as a plain `PathBuf` made it easy for
core/session code to keep fallback normalization and relative-path
resolution logic in places that should only receive an already-resolved
cwd.

This is intentionally the absolute-cwd-only slice: it does not change
environment selection stickiness or cwd-to-default-environment fallback
behavior.

## What changed

- Changes `ThreadSettingsOverrides.cwd`,
`CodexThreadSettingsOverrides.cwd`, and `SessionSettingsUpdate.cwd` to
use `AbsolutePathBuf`.
- Removes core-side cwd normalization/resolution from session settings
updates.
- Updates affected core/app-server test helpers and callsites to pass
existing absolute cwd values or use `abs()` helpers.

## Validation

Opening as draft so CI can start while local validation continues.

codex-rs/core/src/codex_thread.rs

@@ -127,7 +127,7 @@ impl ThreadConfigSnapshot {
 /// Thread settings overrides that app-server validates before starting a turn.
 #[derive(Clone, Default)]
 pub struct CodexThreadSettingsOverrides {
-    pub cwd: Option<PathBuf>,
+    pub cwd: Option<AbsolutePathBuf>,
     pub workspace_roots: Option<Vec<AbsolutePathBuf>>,
     pub profile_workspace_roots: Option<Vec<AbsolutePathBuf>>,
     pub approval_policy: Option<AskForApproval>,