Move shell snapshot scripts into codex-shell-command (#39281)

## What changed

- Add a public `snapshot_script` helper to `codex-shell-command` for Bash,
  Zsh, POSIX shell, and PowerShell, returning `None` for Command Prompt.
- Update `codex-core` and its shell snapshot tests to use the shared helper
  without changing snapshot behavior.

GitOrigin-RevId: 779b9dc964635b717922ea9f5646b94c4dbb0da1

codex-rs/shell-command/src/lib.rs

@@ -1,6 +1,7 @@
 //! Command parsing and safety utilities shared across Codex crates.
 
 pub mod shell_detect;
+pub mod shell_snapshot;
 
 pub mod bash;
 pub(crate) mod command_safety;