fix: Use PreciseTimestamp for create and update time in database session service to improve precision

PiperOrigin-RevId: 794422113

Author: xuanyang15

src/google/adk/sessions/database_session_service.py

@@ -22,7 +22,6 @@
 from typing import Optional
 import uuid
 
-from google.genai import types
 from sqlalchemy import Boolean
 from sqlalchemy import delete
 from sqlalchemy import Dialect
@@ -132,9 +131,11 @@ class StorageSession(Base):
       MutableDict.as_mutable(DynamicJSON), default={}
   )
 
-  create_time: Mapped[DateTime] = mapped_column(DateTime(), default=func.now())
-  update_time: Mapped[DateTime] = mapped_column(
-      DateTime(), default=func.now(), onupdate=func.now()
+  create_time: Mapped[datetime] = mapped_column(
+      PreciseTimestamp, default=func.now()
+  )
+  update_time: Mapped[datetime] = mapped_column(
+      PreciseTimestamp, default=func.now(), onupdate=func.now()
   )
 
   storage_events: Mapped[list[StorageEvent]] = relationship(
@@ -313,8 +314,8 @@ class StorageAppState(Base):
   state: Mapped[MutableDict[str, Any]] = mapped_column(
       MutableDict.as_mutable(DynamicJSON), default={}
   )
-  update_time: Mapped[DateTime] = mapped_column(
-      DateTime(), default=func.now(), onupdate=func.now()
+  update_time: Mapped[datetime] = mapped_column(
+      PreciseTimestamp, default=func.now(), onupdate=func.now()
   )
 
 
@@ -332,8 +333,8 @@ class StorageUserState(Base):
   state: Mapped[MutableDict[str, Any]] = mapped_column(
       MutableDict.as_mutable(DynamicJSON), default={}
   )
-  update_time: Mapped[DateTime] = mapped_column(
-      DateTime(), default=func.now(), onupdate=func.now()
+  update_time: Mapped[datetime] = mapped_column(
+      PreciseTimestamp, default=func.now(), onupdate=func.now()
   )