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/exec-server/src/shell_snapshot.rs

@@ -172,9 +172,17 @@ impl ShellSnapshotCache {
             .collect::<String>();
         let state_variables = state_variables.join(" ");
         let shell_start = prepared.command.len() - params.argv.len();
-        prepared.command[shell_start + 1] = "-c".to_string();
+        // Automatic startup files run before the restoration script and could
+        // reintroduce environment variables that the snapshot already filtered.
+        let (shell_flag, startup) = match shell_type {
+            ShellType::Bash => ("-pc", "set +o privileged\n"),
+            ShellType::Zsh => ("-fc", "setopt RCS\n"),
+            ShellType::Sh => ("-c", ""),
+            ShellType::PowerShell | ShellType::Cmd => unreachable!(),
+        };
+        prepared.command[shell_start + 1] = shell_flag.to_string();
         prepared.command[shell_start + 2] = format!(
-            "if ! eval \"unset {state_variables}\n{state_expansion}\" >/dev/null; then printf 'failed to restore shell snapshot\\n' >&2; fi\n{}",
+            "{startup}if ! eval \"unset {state_variables}\n{state_expansion}\" >/dev/null; then printf 'failed to restore shell snapshot\\n' >&2; fi\n{}",
             params.argv[2]
         );