feat(ext-data): close the connection pool when the app stops

TxManager is AutoCloseable and releases what it was built on: JdbcTxManager its data
source when closeable, HibernateTxManager its session factory and then the data source
Hibernate was handed, which Hibernate itself never closes. DataExtension registers the
close as an onClose callback, so a stopped app no longer leaves its pool connected.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

flash-extensions/flash-ext-data-core/docs/README.md

@@ -112,6 +112,8 @@ public abstract class Repository<T, ID> {
 - `Tx` in the `FlashContext`
 - `TxManager` in the `FlashContext`
 - an annotation processor for `@Transactional`
+- `TxManager.close()` as an `onClose` callback, so stopping the app releases the manager's
+  session factory and connection pool; give the manager a pool you want closed with the app
 
 This makes the data layer composable with Flash's extension system without global state.