Missing `reasoning.mode` (for `pro` mode) setting for GPT 5.6 models

What variant of Codex are you using?

CLI

What feature would you like to see?

Currently, codex has no way of setting the reasoning.mode parameter to pro.

i.e. these should work (at least for those of us using API billing):

codex exec -m gpt-5.6-sol -c model_reasoning_effort=xhigh -c model_reasoning_mode=pro "Hello world"

codex exec -m gpt-5.6-luna -c model_reasoning_effort=medium -c model_reasoning_mode=pro "Hello world"

https://developers.openai.com/api/docs/guides/reasoning#reasoning-mode

Related to (but not a duplicate of, as these issues are not talking about the reasoning _mode_, but rather the older GPT 5.5 Pro model, which does not use reasoning.mode): #27077 and #28709

Additional information

This is a pretty simple fix. (Not going to waste my time doing a PR, since I know OpenAI's current policy is to refuse external PRs.)