fix(harness-codex): support Codex `xhigh` and `max` reasoning levels (#19943)

## Background

The Codex harness restricted `reasoningEffort` to `low`, `medium`, and
`high` in both its public type and bridge validation, rejecting the
supported `xhigh` and `max` values. The currently pinned Codex SDK
already accepts both values, so no dependency update is needed.

## Summary

- Accept `xhigh` and `max` across the public settings, internal session
type, and bridge schema.
- Add type and runtime coverage confirming both values reach Codex as
`modelReasoningEffort`.
- Update the Codex reasoning example, documentation, and patch
changeset.

## Contributor Credit

- @StErMi reported the issue and provided the reproduction details.

## End-to-End Verification

Ran `./tools/run-harness-agent-examples.sh --harness codex --example
with-reasoning` successfully.

## Checklist

- [x] All commits are signed (PRs with unsigned commits cannot be
merged)
- [x] Tests have been added / updated (for bug fixes / features)
- [x] Documentation has been added / updated (for bug fixes / features)
- [x] A _patch_ changeset for relevant packages has been added (for bug
fixes / features - run `pnpm changeset` in the project root)
- [x] I have reviewed this pull request (self-review)

## Related Issues

Fixes #19908

Closes #19910