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/shell-escalation/src/unix/mod.rs

@@ -53,7 +53,6 @@
 //!   |      |
 //!   o<-----x
 //!
-pub mod core_shell_escalation;
 pub mod escalate_client;
 pub mod escalate_protocol;
 pub mod escalate_server;
@@ -62,15 +61,12 @@ pub mod execve_wrapper;
 pub mod socket;
 pub mod stopwatch;
 
-pub use self::core_shell_escalation::ShellActionProvider;
-pub use self::core_shell_escalation::ShellPolicyFactory;
 pub use self::escalate_client::run_shell_escalation_execve_wrapper;
 pub use self::escalate_protocol::EscalateAction;
-pub use self::escalate_server::EscalationPolicyFactory;
+pub use self::escalate_server::EscalateServer;
 pub use self::escalate_server::ExecParams;
 pub use self::escalate_server::ExecResult;
 pub use self::escalate_server::ShellCommandExecutor;
-pub use self::escalate_server::run_escalate_server;
 pub use self::escalation_policy::EscalationPolicy;
 pub use self::execve_wrapper::main_execve_wrapper;
 pub use self::stopwatch::Stopwatch;