[codex] add code-mode host failure supervision hooks (#30110)

## Why

A process host should be discarded and rebuilt after critical actor or
V8 failure, while the existing in-process production path must keep its
current cell-error semantics. This change establishes that failure
boundary without adding the host process or remote client.

## What changed

- add optional task-failure supervision to the transport-neutral
code-mode session runtime
- report Tokio cell-actor failures and V8 runtime-thread panics to a
host-provided fail-stop handler
- preserve the existing handler-less in-process behavior
- make host-owned cell ID allocation fail before numeric wraparound

## Follow-up

The V8 panic signal surfaced here should also be consumed by the
`InProcessCodeModeSession` manager in a future change so it can fail the
affected cell. This PR intentionally leaves the handler-less in-process
behavior unchanged while putting the required panic tracking in place.

## Stack

This is **2 of 4** in the process-owned code-mode session stack.

- #30108 is merged into `main`
- The next PR targets this branch

## Validation

- `just test -p codex-code-mode` — 53 passed
- `just argument-comment-lint -p codex-code-mode`
- `just fix -p codex-code-mode`

codex-rs/code-mode/src/lib.rs

@@ -3,6 +3,8 @@ mod runtime;
 mod service;
 mod session_runtime;
 
+pub(crate) type TaskFailureHandler = std::sync::Arc<dyn Fn(String) + Send + Sync>;
+
 pub use codex_code_mode_protocol::*;
 pub use service::InProcessCodeModeSession;
 pub use service::InProcessCodeModeSessionProvider;