fix: Add close() method to SQLAlchemySession for proper engine disposal

- Add _owns_engine flag to track engine ownership
- from_url() sets _owns_engine=True (SDK creates engine)
- Direct engine injection keeps _owns_engine=False (user manages)
- close() disposes engine only when owned by session
- Matches RedisSession pattern for resource lifecycle management

Fixes resource leak when using from_url() in long-running applications

Author: gn00295120

src/agents/extensions/memory/sqlalchemy_session.py

@@ -81,6 +81,7 @@ def __init__(
         self.session_id = session_id
         self._engine = engine
         self._lock = asyncio.Lock()
+        self._owns_engine = False  # Track if we own the engine
 
         self._metadata = MetaData()
         self._sessions = Table(
@@ -159,7 +160,9 @@ def from_url(
         """
         engine_kwargs = engine_kwargs or {}
         engine = create_async_engine(url, **engine_kwargs)
-        return cls(session_id, engine=engine, **kwargs)
+        session = cls(session_id, engine=engine, **kwargs)
+        session._owns_engine = True  # We created the engine, so we own it
+        return session
 
     async def _serialize_item(self, item: TResponseInputItem) -> str:
         """Serialize an item to JSON string. Can be overridden by subclasses."""
@@ -319,3 +322,13 @@ async def clear_session(self) -> None:
                 await sess.execute(
                     delete(self._sessions).where(self._sessions.c.session_id == self.session_id)
                 )
+
+    async def close(self) -> None:
+        """Close the database engine connection.
+
+        Only disposes the engine if this session owns it
+        (i.e., created via from_url). If the engine was injected externally,
+        the caller is responsible for managing its lifecycle.
+        """
+        if self._owns_engine:
+            await self._engine.dispose()