feat: Add option to send full history to stateless RemoteA2aAgents

Introduces `full_history_when_stateless` to RemoteA2aAgent. When True, stateless agents will receive all session events on each request, instead of only events since their last reply. This allows stateless agents to have access to the complete conversation history.

PiperOrigin-RevId: 854400798

tests/unittests/agents/test_remote_a2a_agent.py

@@ -665,6 +665,157 @@ class TestRemoteA2aAgentMessageHandling:
     assert parts == []
     assert context_id is None
 
+  def test_construct_message_parts_from_session_stops_on_agent_reply(self):
+    """Test message parts construction stops on agent reply by default."""
+    part1 = Mock()
+    part1.text = "User 1"
+    content1 = Mock()
+    content1.parts = [part1]
+    user1 = Mock()
+    user1.content = content1
+    user1.author = "user"
+    user1.custom_metadata = None
+
+    part2 = Mock()
+    part2.text = "Agent 1"
+    content2 = Mock()
+    content2.parts = [part2]
+    agent1 = Mock()
+    agent1.content = content2
+    agent1.author = self.agent.name
+    agent1.custom_metadata = None
+
+    part3 = Mock()
+    part3.text = "User 2"
+    content3 = Mock()
+    content3.parts = [part3]
+    user2 = Mock()
+    user2.content = content3
+    user2.author = "user"
+    user2.custom_metadata = None
+
+    self.mock_session.events = [user1, agent1, user2]
+
+    def mock_converter(part):
+      mock_a2a_part = Mock()
+      mock_a2a_part.text = part.text
+      return mock_a2a_part
+
+    self.mock_genai_part_converter.side_effect = mock_converter
+
+    with patch(
+        "google.adk.agents.remote_a2a_agent._present_other_agent_message"
+    ) as mock_present:
+      mock_present.side_effect = lambda event: event
+      parts, context_id = self.agent._construct_message_parts_from_session(
+          self.mock_context
+      )
+      assert len(parts) == 1
+      assert parts[0].text == "User 2"
+      assert context_id is None
+
+  def test_construct_message_parts_from_session_stateless_full_history(self):
+    """Test full history for stateless agent when enabled."""
+    self.agent._full_history_when_stateless = True
+    part1 = Mock()
+    part1.text = "User 1"
+    content1 = Mock()
+    content1.parts = [part1]
+    user1 = Mock()
+    user1.content = content1
+    user1.author = "user"
+    user1.custom_metadata = None
+
+    part2 = Mock()
+    part2.text = "Agent 1"
+    content2 = Mock()
+    content2.parts = [part2]
+    agent1 = Mock()
+    agent1.content = content2
+    agent1.author = self.agent.name
+    agent1.custom_metadata = None
+
+    part3 = Mock()
+    part3.text = "User 2"
+    content3 = Mock()
+    content3.parts = [part3]
+    user2 = Mock()
+    user2.content = content3
+    user2.author = "user"
+    user2.custom_metadata = None
+
+    self.mock_session.events = [user1, agent1, user2]
+
+    def mock_converter(part):
+      mock_a2a_part = Mock()
+      mock_a2a_part.text = part.text
+      return mock_a2a_part
+
+    self.mock_genai_part_converter.side_effect = mock_converter
+
+    with patch(
+        "google.adk.agents.remote_a2a_agent._present_other_agent_message"
+    ) as mock_present:
+      mock_present.side_effect = lambda event: event
+      parts, context_id = self.agent._construct_message_parts_from_session(
+          self.mock_context
+      )
+      assert len(parts) == 3
+      assert parts[0].text == "User 1"
+      assert parts[1].text == "Agent 1"
+      assert parts[2].text == "User 2"
+      assert context_id is None
+
+  def test_construct_message_parts_from_session_stateful_partial_history(self):
+    """Test partial history for stateful agent when full history is enabled."""
+    self.agent._full_history_when_stateless = True
+    part1 = Mock()
+    part1.text = "User 1"
+    content1 = Mock()
+    content1.parts = [part1]
+    user1 = Mock()
+    user1.content = content1
+    user1.author = "user"
+    user1.custom_metadata = None
+
+    part2 = Mock()
+    part2.text = "Agent 1"
+    content2 = Mock()
+    content2.parts = [part2]
+    agent1 = Mock()
+    agent1.content = content2
+    agent1.author = self.agent.name
+    agent1.custom_metadata = {A2A_METADATA_PREFIX + "context_id": "ctx-1"}
+
+    part3 = Mock()
+    part3.text = "User 2"
+    content3 = Mock()
+    content3.parts = [part3]
+    user2 = Mock()
+    user2.content = content3
+    user2.author = "user"
+    user2.custom_metadata = None
+
+    self.mock_session.events = [user1, agent1, user2]
+
+    def mock_converter(part):
+      mock_a2a_part = Mock()
+      mock_a2a_part.text = part.text
+      return mock_a2a_part
+
+    self.mock_genai_part_converter.side_effect = mock_converter
+
+    with patch(
+        "google.adk.agents.remote_a2a_agent._present_other_agent_message"
+    ) as mock_present:
+      mock_present.side_effect = lambda event: event
+      parts, context_id = self.agent._construct_message_parts_from_session(
+          self.mock_context
+      )
+      assert len(parts) == 1
+      assert parts[0].text == "User 2"
+      assert context_id == "ctx-1"
+
   @pytest.mark.asyncio
   async def test_handle_a2a_response_success_with_message(self):
     """Test successful A2A response handling with message."""