fix(memory): resolve 3 new mypy errors flagged by mypy-diff CI

Addresses the mypy-new-error gate failures on Python 3.10/3.13:

- memory/__init__.py: add `-> Any` return annotation to the lazy
  `__getattr__(name)` so it satisfies `no-untyped-def`. Matches the
  convention used by `tools/__init__.py` and `skills/__init__.py`.
- memory/_memory_schemas.py: silence SQLAlchemy's `DeclarativeBase`
  type-stub `Any` base-class warning (same pattern as other places
  where SQLAlchemy is used; pre-existing analogous errors in
  sessions/schemas/{v0,v1}.py are grandfathered on main).
- memory/database_memory_service.py: annotate `__aexit__` parameters
  so mypy no longer flags `no-untyped-def`.

Verified locally:
  * `mypy .` reports zero errors in the three new files
    (the mypy-diff gate compares against main; all remaining errors
    under `src/google/adk/memory/` are pre-existing on main).
  * All 19 DatabaseMemoryService unit tests still pass.
  * pyink --check and isort --check-only are clean.

Author: anmolg1997

src/google/adk/memory/__init__.py

@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import logging
+from typing import Any
 
 from .base_memory_service import BaseMemoryService
 from .in_memory_memory_service import InMemoryMemoryService
@@ -27,7 +28,7 @@
 ]
 
 
-def __getattr__(name: str):
+def __getattr__(name: str) -> Any:
   if name == 'DatabaseMemoryService':
     try:
       from .database_memory_service import DatabaseMemoryService

src/google/adk/memory/_memory_schemas.py

@@ -32,7 +32,7 @@
 _MAX_KEY_LENGTH = 128
 
 
-class Base(DeclarativeBase):
+class Base(DeclarativeBase):  # type: ignore[misc]
   """Base class for memory database tables."""
 
   pass

src/google/adk/memory/database_memory_service.py

@@ -305,7 +305,13 @@ async def close(self) -> None:
   async def __aenter__(self) -> DatabaseMemoryService:
     return self
 
-  async def __aexit__(self, exc_type, exc_val, exc_tb) -> None:
+  async def __aexit__(
+      self,
+      exc_type: Optional[type[BaseException]],
+      exc_val: Optional[BaseException],
+      exc_tb: Any,
+  ) -> None:
+    """Exits the async context manager and closes the service."""
     await self.close()
 
   # -- keyword helpers ------------------------------------------------------