Fix unrelated capacity assignment blocking checkout

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Summary

• Skip overall capacity checks for capacity assignments that have zero selected products in the current checkout request.
• Add a regression test covering an over-reserved unrelated capacity bucket while the selected product remains available.

Fixes #1195

Tests

• /opt/homebrew/opt/php@8.4/bin/php vendor/bin/phpunit tests/Unit/Services/Domain/Order/OrderCreateRequestValidationServiceTest.php --no-coverage
• /opt/homebrew/opt/php@8.4/bin/php -l app/Services/Domain/Order/OrderCreateRequestValidationService.php
• /opt/homebrew/opt/php@8.4/bin/php -l tests/Unit/Services/Domain/Order/OrderCreateRequestValidationServiceTest.php
• git diff --check

Note

• I also ran /opt/homebrew/opt/php@8.4/bin/php vendor/bin/phpunit tests/Unit --no-coverage locally. It completed with 388 passing tests, 2 skipped tests, and 2 failures in Tests

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