openai_subscribed: Use full context windows for subscription models (#62502)

ChatGPT subscription models currently report the short-context billing
thresholds (272k or 372k tokens) as their maximum context windows. Those
thresholds matter when Zed is paying metered API costs, but subscription
requests are billed directly by OpenAI.

This updates the subscribed models to report the full context windows
supported by the corresponding public API models: 1.05M tokens for
GPT-5.4, GPT-5.5, and GPT-5.6, and 400k tokens for GPT-5.4 Mini. It also
reports the 128k output limit so context accounting reserves capacity
for the response. Request serialization is unchanged; the unsupported
`max_output_tokens` parameter is still omitted from requests to the
Codex backend.

Release Notes:

- Improved context window usage for GPT models accessed through a
ChatGPT subscription.