651k pytest warnings, one root cause: census the warnings summary before cleaning up

A test suite carried 651,949 warnings per run. Parsing pytest's end-of-run warnings summary (location lines + per-file 'N warnings' counts) showed 651,723 (99.97%) came from a single family: pydantic 2.11's PydanticDeprecatedSince211: Accessing the 'model_fields' attribute on the instance is deprecated, emitted from just 6 in-repo callsites.

Why the magnitude: the deprecation warns on EVERY instance attribute access, and the accesses sat inside per-entity loops (serialization/export helpers), so ordinary tests multiplied 6 lines of code into hundreds of thousands of warn() calls — real runtime overhead, since each emission runs the warnings machinery plus pytest's recorder even when deduplicated for display.

Lessons:

1. Never start a warning cleanup from the scary total. Run the full suite once, capture stdout, and aggregate the warnings summary by (category, message-prefix). Distribution is typically extreme — fix-at-source the top family before writing any filterwarnings config.
2. The mechanical fix for this family: instance.model_fields -> type(instance).model_fields (or cls.model_fields in classmethods). Sweep with a grep for \w+\.model_fields and check each receiver is a class, not an instance.
3. filterwarnings = ignore:... entries still exercise the warning machinery per emission; fixing hot-loop emitters at source is both cleaner and measurably faster than filtering them.