refactor: Update Task model fields to use protobuf types for statu…s, artifacts, and history. (#819)

This is a rollback of a change made in:
#783

Author: sokoliva

src/a2a/server/models.py

@@ -11,6 +11,9 @@ def override(func):  # noqa: ANN001, ANN201
         return func
 
 
+from a2a.types.a2a_pb2 import Artifact, Message, TaskStatus
+
+
 try:
     from sqlalchemy import JSON, DateTime, Index, LargeBinary, String
     from sqlalchemy.orm import (
@@ -48,13 +51,11 @@ class TaskMixin:
     last_updated: Mapped[datetime | None] = mapped_column(
         DateTime, nullable=True
     )
-    status: Mapped[dict[str, Any] | None] = mapped_column(JSON, nullable=True)
-    artifacts: Mapped[list[dict[str, Any]] | None] = mapped_column(
-        JSON, nullable=True
-    )
-    history: Mapped[list[dict[str, Any]] | None] = mapped_column(
+    status: Mapped[TaskStatus] = mapped_column(JSON, nullable=False)
+    artifacts: Mapped[list[Artifact] | None] = mapped_column(
         JSON, nullable=True
     )
+    history: Mapped[list[Message] | None] = mapped_column(JSON, nullable=True)
     protocol_version: Mapped[str | None] = mapped_column(
         String(16), nullable=True
     )