ci: Remove volatile metadata from generated models header (#718)

## Summary

- Disable `timestamp` and `filename` in the `datamodel-codegen` header
via `pyproject.toml` config
- Prevents spurious PRs when the OpenAPI spec hasn't changed but
regeneration produces a diff due to volatile metadata (timestamp, source
URL).

## Test plan

- [x] Regenerated models locally — header is now a single static line `#
generated by datamodel-codegen`
- [x] CI passes

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>