feat(e2e): add multi-database E2E tests

Add E2E tests for multi-database registry functionality:
- Database registration and unregistration
- Health checking for single and multiple databases
- SQL dialect adapters (SQLite, PostgreSQL, MySQL, SQL Server)
- Multi-database query routing
- Database isolation verification
- Configuration and lifecycle management

Implements Issue #43 Phase 3.

Author: Sakeeb91