fix(fetch): serialize primitive multipart values

[Suraj-Self (suraj-self)]

Fixes 43185

Dictionary-based multipart data in the Python API silently drops float and bool values.

This change reuses the existing _form_data_field_to_form_field() helper for dictionary-based multipart values, so primitive values are serialized consistently with the existing FormData path.

Added regression coverage for float and bool values in both sync and async multipart tests.

Tests

pytest -q tests/sync/test_fetch_browser_context.py -k "should_support_multipart_form_data"  — 9 passed
pytest -q tests/async/test_fetch_browser_context.py -k "should_support_multipart_form_data" — 9 passed