exec-server: add Noise relay transport (#26242)

## Why

Rendezvous forwards traffic between the orchestrator and exec-server.
The endpoints need to authenticate each other and encrypt that traffic
without trusting Rendezvous with plaintext or endpoint keys.

## Changes

- Adds a hybrid Noise IK channel through Clatter using X25519,
ML-KEM-768, AES-256-GCM, and SHA-256.
- Binds each handshake to `environment_id`, `executor_registration_id`,
and `stream_id`.
- Pins the registry-provided executor key and carries the harness
authorization inside the encrypted handshake.
- Orders relay frames before consuming Noise nonces and fragments large
JSON-RPC messages into bounded records.
- Bounds handshake payloads, frames, streams, and message reassembly.

Runtime activation is in
[openai/codex#26245](https://github.com/openai/codex/pull/26245).

## Stack

1. **[openai/codex#26242](https://github.com/openai/codex/pull/26242)**:
Noise channel and relay transport
2. [openai/codex#26245](https://github.com/openai/codex/pull/26245):
remote registration and runtime activation

## Verification

- `just test -p codex-exec-server`
- Oversized initiator payload regression coverage
- `just fix -p codex-exec-server`
- `just bazel-lock-check`
- `cargo shear`

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/core-api/src/lib.rs

@@ -48,7 +48,12 @@ pub use codex_core::resolve_installation_id;
 pub use codex_core::skills::SkillsManager;
 pub use codex_core::thread_store_from_config;
 pub use codex_exec_server::EnvironmentManager;
+pub use codex_exec_server::ExecServerError;
 pub use codex_exec_server::ExecServerRuntimePaths;
+pub use codex_exec_server::NoiseChannelIdentity;
+pub use codex_exec_server::NoiseChannelPublicKey;
+pub use codex_exec_server::NoiseRendezvousConnectBundle;
+pub use codex_exec_server::NoiseRendezvousConnectProvider;
 pub use codex_extension_api::LoadUserInstructionsFuture;
 pub use codex_extension_api::LoadedUserInstructions;
 pub use codex_extension_api::UserInstructions;