Run exec-server compatibility tests under Bazel (#40736)

## What changed

- Add a Bazel test rule that runs the shared Noise relay compatibility suite with current or packaged Codex binaries.
- Cover both app-server/exec-server directions for the current build, release `0.149.1`, and the minimum supported release `0.145.0` on Linux x86-64.
- Remove the standalone version-skew download scripts and the protocol constant they used.

## Testing

The new `exec-server-current-version-test`, `exec-server-stable-release-test`, and `exec-server-minimum-release-test` targets exercise the compatibility suite.

GitOrigin-RevId: 6b3b91eea8e05a38bec1fac128afc54f0d654014