fix: Enforce foreign key constraint for SQLite DB

Resolves https://github.com/google/adk-python/issues/2752

PiperOrigin-RevId: 801106660

src/google/adk/sessions/database_session_service.py

@@ -26,6 +26,7 @@ import uuid
 from sqlalchemy import Boolean
 from sqlalchemy import delete
 from sqlalchemy import Dialect
+from sqlalchemy import event
 from sqlalchemy import ForeignKeyConstraint
 from sqlalchemy import func
 from sqlalchemy import Text
@@ -366,6 +367,12 @@ class StorageUserState(Base):
   )
 
 
+def set_sqlite_pragma(dbapi_connection, connection_record):
+  cursor = dbapi_connection.cursor()
+  cursor.execute("PRAGMA foreign_keys=ON")
+  cursor.close()
+
+
 class DatabaseSessionService(BaseSessionService):
   """A session service that uses a database for storage."""
 
@@ -374,9 +381,13 @@ class DatabaseSessionService(BaseSessionService):
     # 1. Create DB engine for db connection
     # 2. Create all tables based on schema
     # 3. Initialize all properties
-
     try:
       db_engine = create_engine(db_url, **kwargs)
+
+      if db_engine.dialect.name == "sqlite":
+        # Set sqlite pragma to enable foreign keys constraints
+        event.listen(db_engine, "connect", set_sqlite_pragma)
+
     except Exception as e:
       if isinstance(e, ArgumentError):
         raise ValueError(