chore: refactor exec.rs: create separate seatbelt.rs and spawn.rs files (#1762)

At 550 lines, `exec.rs` was a bit large. In particular, I found it hard
to locate the Seatbelt-related code quickly without a file with
`seatbelt` in the name, so this refactors things so:

- `spawn_command_under_seatbelt()` and dependent code moves to a new
`seatbelt.rs` file
- `spawn_child_async()` and dependent code moves to a new `spawn.rs`
file

codex-rs/mcp-server/tests/interrupt.rs

@@ -3,7 +3,7 @@
 
 use std::path::Path;
 
-use codex_core::exec::CODEX_SANDBOX_NETWORK_DISABLED_ENV_VAR;
+use codex_core::spawn::CODEX_SANDBOX_NETWORK_DISABLED_ENV_VAR;
 use codex_mcp_server::CodexToolCallParam;
 use mcp_types::JSONRPCResponse;
 use mcp_types::RequestId;