fix: Strip timezone for PostgreSQL timestamps in DatabaseSessionService

PostgreSQL's default TIMESTAMP type is WITHOUT TIME ZONE, which cannot
accept timezone-aware datetime objects from Python. This causes a
DataError when using asyncpg: "can't subtract offset-naive and
offset-aware datetimes".

The existing code already handled this for SQLite by stripping the
timezone, but PostgreSQL was not handled. This fix applies the same
treatment to PostgreSQL.

Fixes the regression introduced in commit 1063fa5 which changed from
database-generated timestamps (func.now()) to explicit Python datetimes.

Author: filipecaixeta

src/google/adk/sessions/database_session_service.py

@@ -297,7 +297,8 @@ async def create_session(
       # Store the session
       now = datetime.now(timezone.utc)
       is_sqlite = self.db_engine.dialect.name == "sqlite"
-      if is_sqlite:
+      is_postgres = self.db_engine.dialect.name == "postgresql"
+      if is_sqlite or is_postgres:
         now = now.replace(tzinfo=None)
 
       storage_session = schema.StorageSession(