fix: harden optional connectors and Snowflake references

[skadel]

Summary

• harden optional connector loading and surface actionable CLI/LLM errors
• allow Snowflake operations without a default database when table references are fully qualified
• validate partial Snowflake references early and avoid sending empty database/schema connection parameters
• expand connector and Snowflake schema-import regression coverage and documentation

Validation

uff check build_query/validator.py cli/generate.py cli/main.py models/env_variables.py tests/test_cli_snowflake_schema_import.py utils/snowflake_connector.py

• Python compilation check for the touched Snowflake files
• targeted pytest could not run locally because the repository Python environments are unavailable/broken in this workspace