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/session/session.rs

@@ -1164,7 +1164,21 @@ impl Session {
             } else {
                 shell::default_user_shell()
             };
-            let shell_snapshot = if config.features.enabled(Feature::ShellSnapshot) {
+            let use_executor_shell_snapshots = config.features.enabled(Feature::ShellSnapshotV2)
+                && config.features.enabled(Feature::ShellTool)
+                && config.features.enabled(Feature::UnifiedExec)
+                && matches!(
+                    codex_tools::UnifiedExecShellMode::for_session(
+                        config.features.get(),
+                        crate::tools::tool_user_shell_type(&default_shell),
+                        config.zsh_path.as_ref(),
+                        config.main_execve_wrapper_exe.as_ref(),
+                    ),
+                    codex_tools::UnifiedExecShellMode::Direct
+                );
+            let shell_snapshot = if config.features.enabled(Feature::ShellSnapshot)
+                && !use_executor_shell_snapshots
+            {
                 ShellSnapshot::new(
                     config.codex_home.clone(),
                     thread_id,