refactor: adapt exec-server to shell-escalation executor API

[bolinfest]

Why

This is the exec-server follow-up to the shell-escalation API prep in #12614, split out of PR #12584 to keep review scope smaller.

Separating the exec-server adoption from the core zsh-fork migration makes it easier to review the adapter wiring and confirm this remains behavior-preserving.

What Changed

• Update exec-server imports to the new codex_shell_escalation::unix::* module layout.
• Introduce McpShellCommandExecutor in exec-server/src/unix/mcp.rs to implement shell-escalation's new ShellCommandExecutor trait by delegating to codex_core::exec::process_exec_tool_call.
• Convert codex_core::SandboxState into shell-escalation's crate-local SandboxState before invoking run_escalate_server.
• Update the Unix exec-wrapper entrypoint import in exec-server/src/unix.rs.
• Add tokio-util dependency/lockfile wiring for CancellationToken use in the adapter.

Verification

• No standalone verification run after splitting this adapter change out of the larger zsh-fork feature stack.
• Intended to be behavior-preserving wiring on top of refactor: prepare shell-escalation for reusable executors #12614.

---

Stack created with Sapling. Best reviewed with ReviewStack.

• -> refactor: adapt exec-server to shell-escalation executor API #12615
• refactor: prepare shell-escalation for reusable executors #12614