Test exec-server compatibility across Codex versions (#35990)

## Why

The executor protocol supports Codex releases back to `0.145.0`, so compatibility needs to hold when either side of the app-server/exec-server connection is upgraded first.

## What changed

- Define `MINIMUM_SUPPORTED_CODEX_VERSION` in `codex-exec-server-protocol`.
- Add a Unix test harness that runs current-to-released and released-to-current command execution over authenticated Noise connections.
- Test the current binary against itself, the latest release, and the minimum supported release by default, while allowing explicit release versions.
- Verify that the remote command runs successfully and relay payloads remain encrypted, and increase the relay test timeout to accommodate the end-to-end scenarios.

GitOrigin-RevId: faea8d44fce161f40ed15170876a1282a6de4c22

codex-rs/exec-server-protocol/src/lib.rs

@@ -7,3 +7,6 @@ pub use network_policy::*;
 pub use process_id::ProcessId;
 pub use protocol::*;
 pub use rpc::*;
+
+/// Oldest Codex release supported by the executor protocol.
+pub const MINIMUM_SUPPORTED_CODEX_VERSION: &str = "0.145.0";