Validate dispense BTC quantity

[caydyan]

Supersedes #3387 with the same fix rebased onto the current develop branch.

dispense.compose() did not validate the BTC quantity before checking open dispensers. A negative quantity could bypass the "not enough BTC" guard because must_give becomes negative rather than zero, then the composer returns a destination output with a negative amount.

This PR rejects non-integer and non-positive dispense quantities at the compose validation boundary.

Changes:

• Requires dispense quantity to be an integer.
• Rejects zero or negative dispense quantity.
• Adds regression coverage for negative and non-integer quantities.

Tests:

• python -m pytest counterpartycore/test/units/messages/dispense_test.py::test_compose counterpartycore/test/units/api/compose_test.py::test_compose_dispense -q
• python -m pytest counterpartycore/test/units/messages/dispense_test.py -q
• python -m ruff check counterpartycore/lib/messages/dispense.py counterpartycore/test/units/messages/dispense_test.py
• python -m ruff format --check counterpartycore/lib/messages/dispense.py counterpartycore/test/units/messages/dispense_test.py
• git diff --check

Bounty payment address (BTC): bc1qev5ant33v5y89qqjvcf4mh9hlax5svqf5xd7gc