fix: handle grant column names case-insensitively

[TangoEnSkai]

Resolves #782

Description

Databricks SHOW GRANTS results can expose the documented principal, actionType, and objectType columns with different casing depending on the execution environment. The inherited dbt-spark implementation indexes only Principal, ActionType, and ObjectType, which raises a KeyError when a connector returns lowercase names.

Override grant standardization in the Databricks adapter and resolve the three required columns case-insensitively. Grant filtering and grouping behavior is unchanged.

Checklist

• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have updated the CHANGELOG.md and added information about my change to the "dbt-databricks next" section.
• [Optional] I have run the dbt-databricks-pr-ready project skill for this PR and addressed its merge-readiness feedback

Test plan

• .venv/bin/pytest tests/unit/test_adapter.py::TestDatabricksAdapter::test_standardize_grants_dict_ignores_column_name_case -q (3 passed)
• .venv/bin/pytest tests/unit/test_adapter.py -q (99 passed, 2 skipped)
• .venv/bin/pre-commit run --files dbt/adapters/databricks/impl.py tests/unit/test_adapter.py