openai_subscribed: Restore Codex context limits (#62515)

PR #62502 changed the ChatGPT subscription models to report the
corresponding public API context windows. That was based on a mistaken
assumption: subscription requests go through the separate Codex backend,
which still rejects requests around the previous context limit.

Because the advertised context window also determines when Zed compacts
a conversation, reporting 1.05M tokens delays compaction until after the
Codex backend rejects the request. This reverts #62502 and restores the
previous conservative limits. Longer term, we should load the
account-specific model metadata from the Codex `/models` endpoint rather
than maintaining this list by hand.

Release Notes:

- Fixed automatic context compaction for GPT models accessed through a
ChatGPT subscription.