Verify: Foreign key management

[debba]

Description

Verify that foreign key management works correctly against a real Db2 instance.

Methods to verify

• get_foreign_keys — list foreign keys for a table
• get_all_foreign_keys_batch — return all FK metadata for a schema
• drop_foreign_key — execute ALTER TABLE ... DROP FOREIGN KEY

Acceptance criteria

• Foreign keys are correctly listed with all metadata
• Batch FK retrieval returns complete results
• Drop FK executes successfully
• Error handling for non-existent FKs works

Part of the WIP verification checklist in README.