test(go): enable GetObjects constraint validation tests

[zeroshade]

What's Changed

Enables the primary, foreign, and unique GetObjects constraint validation tests for the Snowflake driver.

Background

The constraint metadata implementation already exists in the SQL-template path of GetObjects: connection.go dispatches SHOW PRIMARY KEYS / SHOW IMPORTED KEYS / SHOW UNIQUE KEYS, and queries/get_objects_all.sql assembles the table_constraints structure. The three validation tests (test_get_objects_constraints_{primary,foreign,unique}) errored with NotImplementedError only because the Snowflake quirks class did not define the required sample_ddl_constraints property.

Changes (go/validation/tests/snowflake.py)

• Add sample_ddl_constraints with Snowflake DDL that creates the primary / unique / foreign constraint fixture tables (modeled on the BigQuery driver's fixtures, adapted to Snowflake syntax — no NOT ENFORCED clause, plus a constraint_unique table since Snowflake supports UNIQUE).
• Enable get_objects_constraints_foreign, get_objects_constraints_primary, and get_objects_constraints_unique.

All identifiers in the DDL are quoted to preserve lower case: Snowflake folds unquoted identifiers to upper case, but the validation suite matches table/column names exactly as written (lower case), consistent with how the suite's adbc_ingest fixtures round-trip.

get_objects_constraints_check stays disabled (Snowflake does not support CHECK constraints). The _normalized quirks stay False because Snowflake preserves the declared key column order (key_sequence), matching the non-normalized branch of the tests.

Note on constraint_column_usage

No driver/SQL change is required. The query emits [] for primary/unique constraint usage, and the driverbase build round-trip (json:"...,omitempty") normalizes that empty list to null — which is exactly what the tests expect (None). Foreign-key usage is populated as before.

Verification

• Quirks load correctly: the three flags are set, sample_ddl_constraints returns 12 statements, and the _normalized quirks are all False.
• pixi run validate --collect-only -k get_objects_constraints collects all four constraint tests; the three enabled ones are active and check is skipped.
• A round-trip unit check confirmed that primary/unique constraint_column_usage renders as null while foreign-key usage stays populated.
• pre-commit (ruff check/format, codespell, Apache RAT) passes on the changed file.
• Full end-to-end validation requires live Snowflake credentials (SNOWFLAKE_URI, SNOWFLAKE_DATABASE, SNOWFLAKE_SCHEMA, and the *_SECONDARY* variants), so the live suite was not executed in this environment.

Closes #48