test: improve unit test coverage and fix deprecation warnings

[kgala2]

Improved unit test coverage to 99% and fixed deprecation warnings for database parameters.

Detailed changes:

• Added unit tests to cover missing statements in almost all modules.
• Updated driver wrappers (asyncpg, pg8000, pymysql, pytds) to support both 'database' and 'db' parameters, ensuring explicit 'database' takes precedence over legacy 'db' to avoid deprecation warnings.
• Updated all system tests to use 'database' parameter instead of 'db' to avoid warnings.
• Added unit tests for all wrappers to verify both 'database' and 'db' parameters work.

New Unit Tests Breakdown (43 tests)

Category	Test Count	% of Total	Key Impact
Connector Lifecycle & Cache	16	37.2%	Ensures background refresh tasks, idempotency, and error-triggered refreshes work cleanly.
HTTP Retries & API Errors	9	20.9%	Protects against transient Cloud SQL Admin API 50x errors and invalid server responses.
Parameter Compatibility	6	14.0%	Prevents deprecation warnings and enforces database precedence.
Security, Auth & SSL/TLS	5	11.6%	Validates token downscoping, IAM username limits, and TLS 1.3 fallback behavior.
Import Error Handling	4	9.3%	Verifies clean ImportError messages for uninstalled database drivers.
DNS & Resolution Guardrails	3	7.0%	Prevents infinite CNAME recursion loops and rejects invalid DNS/connection inputs.
Total	43	100%