Expose CreateMultipartContent as public so the blogs backend tests can
assert on its wire shape directly and round-trip it through the live
BlogsWebServerFixture host. New CreateMultipartContentTests:
- shape: blog part is camelCase JSON with WhenWritingNull, one file
part per attachment (filename, Content-Type, octet-stream fallback);
- round-trip: POST the built body to /api/v1/blogspot, assert 201
Created + attachment row + file landed under BlogFilesRoot.
Co-Authored-By: Claude <noreply@anthropic.com>
