Feat: CXA-1831 Persist latest model and reasoning effort in sqlite (#14859)

### Summary
The goal is for us to get the latest turn model and reasoning effort on
thread/resume is no override is provided on the thread/resume func call.
This is the part 1 which we write the model and reasoning effort for a
thread to the sqlite db and there will be a followup PR to consume the
two new fields on thread/resume.

[part 2 PR is currently WIP](https://github.com/openai/codex/pull/14888)
and this one can be merged independently.

codex-rs/state/migrations/0020_threads_model_reasoning_effort.sql

@@ -0,0 +1,2 @@
+ALTER TABLE threads ADD COLUMN model TEXT;
+ALTER TABLE threads ADD COLUMN reasoning_effort TEXT;