Skip zero balances in sweeps

[caydyan]

Fixes #3089.

This is a develop-based replacement for #3365.

Summary:

• Add a gated sweep_skip_zero_balances protocol change.
• Skip zero-quantity balance rows when sweeping balances after the gate.
• Calculate sweep antispam fees from the balances/ownership categories actually being swept.
• Avoid charging an XCP sweep fee when the gated path has no sweepable balances/ownership.
• Preserve legacy behavior when the gate is disabled.

Tests:

• .venv/bin/python -m pytest counterpartycore/test/units/messages/sweep_test.py -q
• .venv/bin/python -m ruff check counterpartycore/lib/messages/sweep.py counterpartycore/test/units/messages/sweep_test.py
• .venv/bin/python -m ruff format --check counterpartycore/lib/messages/sweep.py counterpartycore/test/units/messages/sweep_test.py
• .venv/bin/python -m json.tool counterpartycore/protocol_changes.json
• git diff --check

BTC payout address if this qualifies for a contributor/security reward: bc1qev5ant33v5y89qqjvcf4mh9hlax5svqf5xd7gc