[codex] Reject unsupported entry creation wrappers

[Flerpharos]

Summary

• reject unsupported fallback entry wrappers before creating entries
• prevent internal sentinel part types from being sent to entries/add_entry
• cover UnknownEntry, UnimplementedEntry, and WidgetEntry with no-API-call tests

Fixes #197

Validation

• uv run --python 3.10 pytest --no-cov tests\entry\test_collection.py
• uv run --python 3.10 pre-commit run --all-files
• uv run --python 3.10 pre-commit run --all-files --hook-stage pre-push