[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/core/src/unified_exec/process_manager.rs

@@ -57,6 +57,7 @@ use codex_protocol::protocol::ExecCommandSource;
 use codex_tools::ToolName;
 use codex_utils_absolute_path::AbsolutePathBuf;
 use codex_utils_output_truncation::approx_token_count;
+use codex_utils_path_uri::PathUri;
 
 const UNIFIED_EXEC_ENV: [(&str, &str); 10] = [
     ("NO_COLOR", "1"),
@@ -156,7 +157,7 @@ fn exec_server_params_for_request(
     codex_exec_server::ExecParams {
         process_id: exec_server_process_id(process_id).into(),
         argv: request.command.clone(),
-        cwd: request.cwd.to_path_buf(),
+        cwd: PathUri::from_abs_path(&request.cwd),
         env_policy,
         env,
         tty,

codex-rs/core/src/unified_exec/process_manager_tests.rs

@@ -66,7 +66,7 @@ fn env_overlay_for_exec_server_keeps_runtime_changes_only() {
 }
 
 #[test]
-fn exec_server_params_use_env_policy_overlay_contract() {
+fn exec_server_params_use_path_uri_and_env_policy_overlay_contract() {
     let cwd: codex_utils_absolute_path::AbsolutePathBuf = std::env::current_dir()
         .expect("current dir")
         .try_into()
@@ -115,6 +115,7 @@ fn exec_server_params_use_env_policy_overlay_contract() {
         exec_server_params_for_request(/*process_id*/ 123, &request, /*tty*/ true);
 
     assert_eq!(params.process_id.as_str(), "123");
+    assert_eq!(params.cwd, PathUri::from_abs_path(&request.cwd));
     assert!(params.env_policy.is_some());
     assert_eq!(
         params.env,