transformers

10050aaf - Exporters: simplify the artifacts API after review

Exporters: simplify the artifacts API after review `ExportFormat` is a `StrEnum`, so configs serialize as they are and custom backends use plain strings. The registry lookup splits into `get_export_config_class` and `get_exporter_class`. The metadata drops the fields that described the traced program rather than the saved graph. Overlapping tests are merged. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>