Add doc test CI workflow reusing existing model job infrastructure
- Add `test_path_prefix` and `test_file_suffix` optional inputs to `model_jobs.yml` and `self-scheduled.yml` (backward-compatible, defaults preserve existing behavior)
- Extend `set-matrix` in `self-scheduled.yml` to discover runnable doc examples when `test_file_suffix` is set, emitting one slice per model for full parallelism
- Add `self-doc-scheduled-caller.yml` to run doc tests daily, reusing `run_models_gpu` + all existing Slack/failure-check infrastructure
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>