test: add regression coverage for list-of-primitive multipart fields

[kraenhansen]

Summary

Adds tests/test_multipart_list_fields.py, a regression test for the fix in #825 (which resolved #819).

The bug: generator fern-python-sdk 4.46.9 began serializing every non-scalar multipart field via json.dumps(jsonable_encoder(...)). List[str] is not a scalar, so speech_to_text.convert(keyterms=["a", "b"]) was sent as one form field containing ["a", "b"] instead of two keyterms fields, and the API validated the whole JSON array against its per-item limits (invalid_keyword_length).

Because the fix lives in Fern-generated raw_client.py files and is re-applied on every regeneration by Fern Replay, there was nothing stopping a future regeneration from silently reverting it. These tests assert the wire format directly, so a revert fails CI.

How it works

Each test builds a real ElevenLabs / AsyncElevenLabs client backed by an httpx.MockTransport whose handler records the outgoing request and raises a sentinel to abort before a response is needed. The captured multipart body is parsed with email.message_from_bytes into an ordered list of (name, value) pairs, and the tests assert on those pairs.

Coverage

All eight fields restored by #825, plus the async path and a negative control:

Test target	Field(s)
speech_to_text.convert (sync + async)	keyterms
dubbing.project.create	keyterms
music.video_to_music	tags
music.finetunes.create	tags
studio.projects.create	genres, pronunciation_dictionary_locators, voice_settings
audio_native.create	pronunciation_dictionary_locators

test_speech_to_text_object_fields_stay_json_encoded is the negative control: object-valued fields such as entity_detection must keep json.dumps encoding, so the tests would also catch an over-broad "fix" that unwrapped those too.

Test plan

• pytest tests/test_multipart_list_fields.py — 9 passed
• Reverting src/elevenlabs/ to the commit before fix: send list-of-primitive multipart fields as repeated form fields (#819) #825 makes all 8 wire-format tests fail (the object-field control still passes, as expected)
• ruff@0.11.5 check + format and mypy@1.13.0 clean on the new file
• No live API key or network access required

Notes

The tests only touch tests/, which is listed in .fernignore, so nothing here is at risk of being overwritten by regeneration.