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/turn_context.rs

@@ -42,6 +42,7 @@ pub(crate) struct TurnEnvironment {
     pub(crate) environment: Arc<Environment>,
     pub(crate) shell: Option<shell::Shell>,
     pub(crate) shell_snapshot: ShellSnapshotTask,
+    pub(crate) shell_snapshot_v2_supported: bool,
 }
 
 impl TurnEnvironment {
@@ -58,6 +59,7 @@ impl TurnEnvironment {
             environment,
             shell,
             shell_snapshot: futures::future::ready(None).boxed().shared(),
+            shell_snapshot_v2_supported: false,
         }
     }