[CUBRID] Drop foreign keys before clean

[Srltas]

Summary

Flyway clean fails on CUBRID schemas containing tables linked by foreign keys. CUBRID prevents dropping a table that is still referenced by a foreign key from another table, so the current CubridSchema.doClean() fails when a parent table is encountered before its referecing children, and cannot resolve circular foreign key references at all.

This PR fixes ths issue by discovering incoming foreign keys via JDBC metadata and dropping those constraints before dropping the tables.

Details

• CubridSchema.doClean() now calls dropForeignKeys() before iterating over doAllTables().
• dropForeignKeys() queries DatabaseMetaData.getImportedKeys() for each table in the schema and runs ALTER TABLE ... DROP CONSTRAINT for every incoming foreign key.
• With the constraints removed first, the subsequent DROP TABLE sequence succeeds regardless of order.
• Adds cleanTablesWithForeignKeys intergration test that creates parent_table and child_table linked by a foreign key, then verifies flyway.clean() removes both.

Validation

Tested locally with Java 21:

sh ./mvnw -pl flyway-database-cubrid test

Result:

Tests run: 7, Failures: 0, Errors: 0, Skipped: 0
BUILD SUCCESS