Adds a deterministic test for the rawEffort resolution chain in
open-sse/executors/codex.ts so a future refactor that flips the priority
back (the bug we just fixed) trips an explicit assertion. Also verifies
end-to-end behavior of the priority order: modelEffort > explicitReasoning
> requestReasoningEffort > fallbackReasoningEffort.
The source fix itself landed earlier on this branch; this commit only
adds the test coverage.
