fix(datasource): dispose SQLAlchemy engine when removing pool

[ashen-forest]

What

Fixes #1358.

ConnectionPoolManager caches SQLAlchemy sessionmaker factories, but the eviction, remove_pool(), and close_all() paths call .close() on those factories. sessionmaker does not expose an instance close() method, so datasource updates can fail with:

AttributeError: 'sessionmaker' object has no attribute 'close'

This change keeps the public behavior of get_pool() unchanged while storing both the session factory and its bound Engine in the LRU cache. Pool cleanup now calls Engine.dispose(), which is responsible for disposing the underlying SQLAlchemy connection pool.

Changes

• Keep (session_factory, engine) in ConnectionPoolManager cache entries.
• Return the same cached sessionmaker factory to callers.
• Dispose the bound engine on LRU eviction, remove_pool(), and close_all().
• Add regression tests for all three cleanup paths.

Tests

Added regression coverage for:

• explicit pool removal
• LRU pool eviction
• closing all managed pools