code-mode: introduce durable session interface (#24180)

## Summary

Introduce a `CodeModeSession` interface for executing and managing
code-mode cells.

This moves cell lifecycle, callback delegation, termination, and
shutdown behind a session abstraction, while continuing to use the
existing in-process implementation, and the ability to implement an
external process one behind this interface.

A Codex session owns one `CodeModeSession`, which in turn owns its
running cells and stored code-mode state. Each cell is represented to
the caller as a `StartedCell`, exposing its cell ID and initial
response.

It also introduces a `CodeModeSessionDelegate` callback interface. A
session uses the delegate to invoke nested host tools and emit
notifications while a cell is running, allowing the runtime to
communicate with its owning Codex session without depending directly on
core turn handling.

<img width="2121" height="1001" alt="image"
src="https://github.com/user-attachments/assets/c349a819-2a59-485c-bda4-2caf68ac4c31"
/>

codex-rs/code-mode/Cargo.toml

@@ -16,8 +16,6 @@ sandbox = ["v8/v8_enable_sandbox"]
 workspace = true
 
 [dependencies]
-async-channel = { workspace = true }
-async-trait = { workspace = true }
 codex-protocol = { workspace = true }
 deno_core_icudata = { workspace = true }
 serde = { workspace = true, features = ["derive"] }