Add in-memory shell snapshots to unified exec (#39957)

## What changed

- Add the under-development `shell_snapshot_v2` feature and advertise executor support through environment capabilities.
- Use executor-managed, in-memory snapshots for eligible direct `bash`, `zsh`, and `sh` login commands, while applying the configured shell environment policy and avoiding duplicate automatic startup-file effects.
- Keep file-backed snapshots available for user-shell commands and fall back to the existing execution path when in-memory snapshots are unsupported or inapplicable.

## Testing

- Cover policy filtering, snapshot reuse without snapshot files, automatic startup files, local and remote execution, and legacy user-shell snapshots.

GitOrigin-RevId: 4ad6cdf13824913ac8c393ba38b9844230677579

codex-rs/core/src/exec.rs

@@ -408,6 +408,7 @@ pub(crate) async fn execute_exec_request(
         cwd,
         env,
         exec_server_env_config: _,
+        exec_server_shell_snapshot: _,
         network,
         expiration,
         capture_policy,