Validate dust size parameters

[caydyan]

Supersedes #3382 with the same fix rebased onto the current develop branch.

Deprecated but still-active compose parameters regular_dust_size and multisig_dust_size accepted negative values. Those values are used directly when creating default destination/change/data outputs, so a negative dust size can become an invalid transaction output amount.

This PR rejects negative dust size overrides with a clear ComposeError.

Changes:

• Rejects negative regular_dust_size.
• Rejects negative multisig_dust_size.
• Adds regression coverage for both parameters while preserving existing default and positive override behavior.

Tests:

• python -m pytest counterpartycore/test/units/api/composer_test.py::test_dust_size -q
• python -m pytest counterpartycore/test/units/api/composer_test.py -q
• python -m ruff check counterpartycore/lib/api/composer.py counterpartycore/test/units/api/composer_test.py
• python -m ruff format --check counterpartycore/lib/api/composer.py counterpartycore/test/units/api/composer_test.py
• git diff --check

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