Add exec-server exec RPC implementation (#15090)

Stacked PR 2/3, based on the stub PR.

Adds the exec RPC implementation and process/event flow in exec-server
only.

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

codex-rs/exec-server/src/server.rs

@@ -1,6 +1,7 @@
+mod filesystem;
 mod handler;
-mod jsonrpc;
 mod processor;
+mod registry;
 mod transport;
 
 pub(crate) use handler::ExecServerHandler;