fix: send list-of-primitive multipart fields as repeated form fields (#819) (#825)

Interim hotfix for the v2.59.0 regression introduced by generator
fern-python-sdk 4.46.9 (commit cb4feb824, PR fern-api/fern#11284), which
began serializing every non-scalar multipart form field via
json.dumps(jsonable_encoder(...)). List[str] fields are not scalar
primitives, so they were sent as a single JSON-array string instead of
repeated form fields -- causing the API to validate the whole array
against per-item limits (e.g. speech_to_text keyterms ->
invalid_keyword_length).

Reverts the 8 list-of-primitive fields to raw values so httpx emits
repeated form fields, matching generator fix 5.22.2. Object fields
(labels, entity_detection, entity_redaction, webhook_metadata) keep
json.dumps. Removes now-unused json / jsonable_encoder imports.

This is intended to be carried across regenerations by Fern Replay and
retired (`fern replay forget`) once the 4.x backport of 5.22.2 lands.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>