Codex model list is missing newer models (gpt-5.6)

The codex model picker only offers GPT-5.5, GPT-5.4 and GPT-5.4-Mini. The gpt-5.6 models never appear, even with an up to date Codex CLI.

The models endpoint gates what it returns on the client_version query parameter, and _codex_models_client_version() returns the first value it finds, checking models_cache.json before version.json. On this machine:

~/.codex/models_cache.json   client_version 0.134.0   (stale, written months ago)
~/.codex/version.json        latest_version 0.146.0
codex --version                             0.146.0

So it reports 0.134.0. Asking the endpoint with each value:

client_version=0.134.0 -> gpt-5.5, gpt-5.4, gpt-5.4-mini
client_version=0.146.0 -> gpt-5.6-sol, gpt-5.6-terra, gpt-5.6-luna, gpt-5.5, gpt-5.4, gpt-5.4-mini

The stale cache pins the list, so the newer models are hidden. Without any Codex CLI installed it is worse: the CODEX_MODELS_CLIENT_VERSION fallback is 0.133.0, which predates gpt-5.6 entirely.

The models do work once they are reachable. Sending the plugin's exact request payload for gpt-5.6-sol and gpt-5.6-terra returns HTTP 200 with the same event types _normalize_response already parses, so nothing else needs to change.