fix: make DatabaseSessionService visible to API documentation (#4331)

src/google/adk/sessions/__init__.py

@@ -14,6 +14,7 @@
 
 from __future__ import annotations
 
+from typing_extensions import override
 import importlib
 from typing import TYPE_CHECKING
 
@@ -28,6 +29,45 @@
   from .in_memory_session_service import InMemorySessionService
   from .vertex_ai_session_service import VertexAiSessionService
 
+try:
+  from .database_session_service import DatabaseSessionService
+except ImportError:
+  # This handles the case where optional dependencies (like sqlalchemy)
+  # are not installed. A placeholder class ensures the symbol is always
+  # available for documentation tools and static analysis.
+  # We use type: ignore[no-redef, misc] to satisfy strict mypy checks.
+  class DatabaseSessionService(BaseSessionService):  # type: ignore[no-redef, misc]
+    """Placeholder for DatabaseSessionService when dependencies are not installed."""
+
+    _ERROR_MESSAGE = (
+        'DatabaseSessionService requires sqlalchemy>=2.0, please ensure it is'
+        ' installed correctly.'
+    )
+
+    def __init__(self, *args: Any, **kwargs: Any) -> None:
+      raise ImportError(self._ERROR_MESSAGE)
+
+    @override
+    async def create_session(self, *args: Any, **kwargs: Any) -> Any:
+      raise ImportError(self._ERROR_MESSAGE)
+
+    @override
+    async def get_session(self, *args: Any, **kwargs: Any) -> Any:
+      raise ImportError(self._ERROR_MESSAGE)
+
+    @override
+    async def list_sessions(self, *args: Any, **kwargs: Any) -> Any:
+      raise ImportError(self._ERROR_MESSAGE)
+
+    @override
+    async def delete_session(self, *args: Any, **kwargs: Any) -> Any:
+      raise ImportError(self._ERROR_MESSAGE)
+
+    @override
+    async def append_event(self, *args: Any, **kwargs: Any) -> Any:
+      raise ImportError(self._ERROR_MESSAGE)
+
+
 __all__ = [
     'BaseSessionService',
     'DatabaseSessionService',
@@ -55,3 +95,4 @@ def __getattr__(name: str):
       raise missing_extra('sqlalchemy', 'db') from e
     return vars(module)['DatabaseSessionService']
   raise AttributeError(f'module {__name__!r} has no attribute {name!r}')
+