Fix Snowflake FK metadata parent/child direction

[ianu82]

Summary

• fix meta_get_foreign_keys column renaming so Snowflake PK columns map to parent_* and FK columns map to child_*
• extend the unit test to assert actual parent/child table+column values, not only column presence

Why

SHOW IMPORTED KEYS returns primary-key metadata in pk_* columns and foreign-key metadata in fk_* columns. The previous rename inverted this relationship, producing reversed lineage in the catalog.

Validation

• python3 -m py_compile mindsdb/integrations/handlers/snowflake_handler/snowflake_handler.py tests/unit/handlers/test_snowflake.py
• pytest -q tests/unit/handlers/test_snowflake.py -k meta_get_foreign_keys_filters (fails in this environment due to missing optional dependency appdirs)

[github-actions]

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[StpMax]

I can't check it, but looks convincing.
Will be also good to change columns names in the test