Fix incorrect imports and add missing getNumberOfEnemies method in enemy versions 6-9

[darbyluv2code]

Summary

• Fix incorrect imports in Implementation.java for enemyversion6, 7, 8, 9
• Each version was importing from oop.enemyversion5 instead of its own package
• Add missing getNumberOfEnemies() static method to Enemy.java in all 4 versions
• Resolves compilation errors in GitHub Actions build

Changes

• enemyversion6: Fixed import to use oop.enemyversion6, added getter method
• enemyversion7: Fixed import to use oop.enemyversion7, added getter method
• enemyversion8: Fixed import to use oop.enemyversion8, added getter method
• enemyversion9: Fixed import to use oop.enemyversion9.enemies, added getter method

Test plan

• Verify mvn clean compile succeeds for all 4 projects
• Verify GitHub Actions build passes for Section 06