Fix multi-connection ORM state handling and prevent non-column attribute leaks

[techmahedy]

• Adds focused coverage for connection pooling and multi-database model CRUD, including fetch, create, update, delete, explicit connection queries, and connection-scoped isolation.
• Fixes runtime connection propagation so hydrated and builder-created models keep the connection they were loaded from, avoiding writes drifting back to the default connection.
• Prevents attributes that do not exist as real table columns from surviving save() and appearing in serialized model output.
• Hardens column metadata caching to use the active PDO instance, eliminating cross-test schema leakage and restoring a fully green suite.