[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/processor.rs

@@ -195,6 +195,7 @@ mod tests {
     use codex_app_server_protocol::JSONRPCRequest;
     use codex_app_server_protocol::JSONRPCResponse;
     use codex_app_server_protocol::RequestId;
+    use codex_utils_path_uri::PathUri;
     use serde::Serialize;
     use serde::de::DeserializeOwned;
     use tokio::io::AsyncBufReadExt;
@@ -396,7 +397,7 @@ mod tests {
         ExecParams {
             process_id,
             argv: sleep_then_print_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,
             tty: false,