fix: Resolve multiple bugs in SnowflakeRegistry and Snowflake connection handling

[jials]

What this PR does / why we need it:

• Fix init ordering and infinite recursion in SnowflakeRegistry caused by premature attribute access during __init__
• Fix column casing bugs where Snowflake's uppercase column names broke metadata sync
• Fix set type hints — use set() instead of [] for set-typed variables in _sync_feast_metadata_to_projects_table
• Remove leftover private key kwargs (private_key, private_key_path, private_key_passphrase) before passing to snowflake.connector.connect, preventing unexpected keyword argument errors
• Resolve mypy type errors in SnowflakeRegistry cache methods
• Filter empty SQL commands at execute_snowflake_statement call sites to avoid execution errors
• Correct project_id scoping in get_user_metadata and delete_project
• Add snowflake_registry fixture to integration registry tests
• Add comprehensive unit tests for SnowflakeRegistry with xdist_group isolation
• Add snowflake_registry to CI filter and exclude from non-Snowflake test targets

Which issue(s) this PR fixes:

Fixes #5204

Checks

• I've made sure the tests are passing.
• My commits are signed off (git commit -s)
• My PR title follows conventional commits format

Testing Strategy

• Unit tests
• Integration tests
• Manual tests
• Testing is not required for this change

Misc

Key files changed:

• sdk/python/feast/infra/registry/snowflake.py — core bug fixes
• sdk/python/feast/infra/utils/snowflake/snowflake_utils.py — private key kwarg cleanup
• sdk/python/tests/unit/infra/registry/test_snowflake_registry.py — new unit test suite
• sdk/python/tests/integration/registration/test_universal_registry.py — integration fixture
• Makefile / .github/workflows/ — CI configuration for snowflake registry tests