fix: make EscalateServer public and remove shell escalation wrappers (#12724)

## Why

`codex-shell-escalation` exposed a `codex-core`-specific adapter layer
(`ShellActionProvider`, `ShellPolicyFactory`, and `run_escalate_server`)
that existed only to bridge `codex-core` to `EscalateServer`. That
indirection increased API surface and obscured crate ownership without
adding behavior.

This change moves orchestration into `codex-core` so boundaries are
clearer: `codex-shell-escalation` provides reusable escalation
primitives, and `codex-core` provides shell-tool policy decisions.

Admittedly, @pakrym rightfully requested this sort of cleanup as part of
https://github.com/openai/codex/pull/12649, though this avoids moving
all of `codex-shell-escalation` into `codex-core`.

## What changed

- Made `EscalateServer` public and exported it from `shell-escalation`.
- Removed the adapter layer from `shell-escalation`:
  - deleted `shell-escalation/src/unix/core_shell_escalation.rs`
- removed exports for `ShellActionProvider`, `ShellPolicyFactory`,
`EscalationPolicyFactory`, and `run_escalate_server`
- Updated `core/src/tools/runtimes/shell/unix_escalation.rs` to:
  - create `Stopwatch`/cancellation in `codex-core`
  - instantiate `EscalateServer` directly
  - implement `EscalationPolicy` directly on `CoreShellActionProvider`

Net effect: same escalation flow with fewer wrappers and a smaller
public API.

## Verification

- Manually reviewed the old vs. new escalation call flow to confirm
timeout/cancellation behavior and approval policy decisions are
preserved while removing wrapper types.

codex-rs/Cargo.lock

@@ -2241,7 +2241,6 @@ dependencies = [
  "anyhow",
  "async-trait",
  "clap",
- "codex-execpolicy",
  "libc",
  "path-absolutize",
  "pretty_assertions",