Route unified exec and filesystem tools through exec-server

Add exec-server filesystem RPCs and a core-side remote filesystem client,
then route unified-exec and filesystem-backed tools through that backend
when enabled by config. Also add Docker-backed remote exec integration
coverage for the local codex-exec CLI.

Co-authored-by: Codex <noreply@openai.com>

codex-rs/exec/tests/suite/mod.rs

@@ -6,6 +6,7 @@ mod ephemeral;
 mod mcp_required_exit;
 mod originator;
 mod output_schema;
+mod remote_exec_server;
 mod resume;
 mod sandbox;
 mod server_error_exit;