Write split db-report downloads as a zip

[jiatolentino]

Summary

dm discover db-report now handles the split-zip format: when the server returns a zip of CSV parts, the CLI writes it as a binary .zip instead of mangling it as text.

Background

Large discovery reports are now split server-side into a zip of numbered CSV parts (datamasque DM-3165). datamasque-python's get_db_discovery_result_report returns bytes for that case.

What changed

• db-report branches on the client return type:
  • str (CSV) → unchanged (write to -o, or echo to stdout).
  • bytes (zip) → write the binary archive, correcting a non-.zip output path to .zip. Requires -o (a binary zip can't be streamed to stdout); aborts with a clear hint otherwise.

Testing

• pytest tests/commands/test_discovery.py — added split-zip and no-output abort tests.
• ruff, ruff format, mypy clean.

Dependency

• Depends on datamasque-python returning bytes for split reports (companion PR). Bump the minimum datamasque-python pin once that release is cut — otherwise an older client returns corrupt text against a new server.