code-mode standalone: extract protocol and add host crate (#27724)

This is phase 1 of a 4 phase stack:
1. **Add protocol and host crates for new IPC code mode implementation**
2. Create the new standalone binary
3. Create a new IPC `CodeModeSessionProvider` to use new binary
4. Remove v8 from core and only use IPC provider


## Add protocol and host crates for new IPC code mode implementation
Establish a clean process boundary without changing the existing
in-process behavior.

- Add the codex-code-mode-protocol crate for shared session, runtime,
response, and tool-definition types.
- Move protocol-facing code out of the V8-backed implementation.
- Add a buildable codex-code-mode-host crate as the foundation for the
standalone process.
- Keep the existing in-process runtime as the active implementation.

codex-rs/code-mode-protocol/Cargo.toml

@@ -0,0 +1,24 @@
+[package]
+edition.workspace = true
+license.workspace = true
+name = "codex-code-mode-protocol"
+version.workspace = true
+
+[lib]
+doctest = false
+name = "codex_code_mode_protocol"
+path = "src/lib.rs"
+
+[lints]
+workspace = true
+
+[dependencies]
+codex-protocol = { workspace = true }
+serde = { workspace = true, features = ["derive"] }
+serde_json = { workspace = true }
+tokio = { workspace = true, features = ["io-util", "sync"] }
+tokio-util = { workspace = true, features = ["rt"] }
+
+[dev-dependencies]
+pretty_assertions = { workspace = true }
+tokio = { workspace = true, features = ["macros", "rt-multi-thread"] }