[codex] define code mode host handshake protocol (#29515)

## Summary

- add validated protocol-version, capability, and session identifier
types
- define explicit `ClientToHost` and `HostToClient` JSON envelopes for
connection negotiation and session open/close acknowledgements
- reject invalid states and unknown fields during decoding, with
explicit wire-format and round-trip coverage

## Why

This establishes the transport-neutral encoding shape needed to build
and test the new code-mode host incrementally. Cell, tool callback, and
failure-domain messages are intentionally deferred until their actors
and behavior tests establish the required semantics.

This is additive protocol scaffolding and does not change the current
production code-mode implementation.

## Validation

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

@@ -1,4 +1,5 @@
 mod description;
+pub mod host;
 mod response;
 mod runtime;
 mod session;