[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/environment.rs

@@ -498,6 +498,7 @@ mod tests {
     use crate::ProcessId;
     use crate::environment_provider::EnvironmentDefault;
     use crate::environment_provider::EnvironmentProviderSnapshot;
+    use codex_utils_path_uri::PathUri;
     use pretty_assertions::assert_eq;
 
     fn test_runtime_paths() -> ExecServerRuntimePaths {
@@ -862,7 +863,8 @@ mod tests {
             .start(crate::ExecParams {
                 process_id: ProcessId::from("default-env-proc"),
                 argv: vec!["true".to_string()],
-                cwd: std::env::current_dir().expect("read current dir"),
+                cwd: PathUri::from_path(std::env::current_dir().expect("read current dir"))
+                    .expect("cwd URI"),
                 env_policy: None,
                 env: Default::default(),
                 tty: false,