[codex] migrate CircleCI Docker executors to gen2

[satanTime]

What changed

Migrates every CircleCI Docker executor in .circleci/config.yml from resource_class: medium to resource_class: medium.gen2.

The Windows orb job is unchanged because it does not use the Docker executor resource classes.

Why

This lets the whole CircleCI Docker CI matrix run on CircleCI's gen2 Docker compute so the project can benchmark the real runtime and credit impact.

CircleCI currently prices medium.gen2 higher per minute than medium, so this PR should be evaluated using the actual CI run duration and consumed credits before merging.

Validation

• git diff --check
• Ruby YAML parse of .circleci/config.yml
• Confirmed no remaining resource_class: medium entries in .circleci/config.yml

CircleCI CLI validation was not run because the circleci CLI is not installed locally.