Route unified exec process startup through remote exec-server

Add experimental config for a websocket exec-server endpoint, select a remote backend at the unified-exec process creation seam, and wrap remote sessions behind the existing process abstraction so exec_command can stream output, write stdin, and terminate over the current exec-server protocol.

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;