Type Cloud Reco multipart fields

[adamtheturtle]

Summary

• build Cloud Reco multipart fields with urllib3 RequestField values
• serialize the numeric result limit before multipart encoding
• remove the broad Any annotations and Pyrefly suppressions without changing the encoded request

Validation

• compared the previous and new multipart bytes with a fixed boundary
• prek run --all-files
• prek run --all-files --hook-stage pre-push
• uv run --group=dev pytest -q --cov-fail-under 100 --cov=src/ --cov=tests/ . (460 passed, 100% coverage)