exec-server: use jsonrpsee for RPC dispatch

Rewrite the transport request/response path to register exec-server methods on a jsonrpsee RpcModule instead of the custom JSON-RPC router/processor, while keeping the in-process client on direct typed handler calls.

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

codex-rs/exec-server/Cargo.toml

@@ -17,6 +17,7 @@ clap = { workspace = true, features = ["derive"] }
 codex-app-server-protocol = { workspace = true }
 codex-utils-pty = { workspace = true }
 futures = { workspace = true }
+jsonrpsee = { workspace = true }
 serde = { workspace = true, features = ["derive"] }
 serde_json = { workspace = true }
 thiserror = { workspace = true }