[codex] Carry exec-server cwd as PathUri (#28032)

## Why

This is the second-to-last place in the exec-server protocol that needs
to migrate to URIs to support cross-OS operation.

## What

- Change `ExecParams.cwd` to `PathUri`.
- Keep the cwd URI-shaped through core and rmcp producers, converting it
to `AbsolutePathBuf` only in `LocalProcess::start_process`.
- Reject non-native cwd URIs before launch and update the affected
protocol documentation and call sites.

codex-rs/exec-server/src/server/handler/tests.rs

@@ -2,6 +2,7 @@ use std::collections::HashMap;
 use std::sync::Arc;
 use std::time::Duration;
 
+use codex_utils_path_uri::PathUri;
 use pretty_assertions::assert_eq;
 use tokio::sync::mpsc;
 use uuid::Uuid;
@@ -26,7 +27,7 @@ fn exec_params_with_argv(process_id: &str, argv: Vec<String>) -> ExecParams {
     ExecParams {
         process_id: ProcessId::from(process_id),
         argv,
-        cwd: std::env::current_dir().expect("cwd"),
+        cwd: PathUri::from_path(std::env::current_dir().expect("cwd")).expect("cwd URI"),
         env_policy: None,
         env: inherited_path_env(),
         tty: false,