feat: Add A2A (Agent-to-Agent) protocol support for remote interoperability

- Implement CrewAgentExecutor class that wraps CrewAI crews as A2A-compatible agents
- Add server utilities for starting A2A servers with crews
- Include comprehensive test coverage for all A2A functionality
- Add optional dependency group 'a2a' in pyproject.toml
- Expose A2A classes in main CrewAI module with graceful import handling
- Add documentation and examples for A2A integration
- Support bidirectional agent communication via A2A protocol
- Enable crews to participate in remote agent networks

Fixes #2970

Co-Authored-By: João <joao@crewai.com>

tests/a2a/test_server.py

@@ -0,0 +1,129 @@
+"""Tests for A2A server utilities."""
+
+import pytest
+from unittest.mock import Mock, patch
+
+try:
+    from crewai.a2a import start_a2a_server, create_a2a_app
+    from a2a.server.agent_execution.agent_executor import AgentExecutor
+    A2A_AVAILABLE = True
+except ImportError:
+    A2A_AVAILABLE = False
+
+
+@pytest.mark.skipif(not A2A_AVAILABLE, reason="A2A integration not available")
+class TestA2AServer:
+    """Test cases for A2A server utilities."""
+    
+    @pytest.fixture
+    def mock_agent_executor(self):
+        """Create a mock AgentExecutor."""
+        return Mock(spec=AgentExecutor)
+    
+    @patch('uvicorn.run')
+    @patch('crewai.a2a.server.create_a2a_app')
+    def test_start_a2a_server_default(self, mock_create_app, mock_uvicorn_run, mock_agent_executor):
+        """Test starting A2A server with default parameters."""
+        mock_app = Mock()
+        mock_create_app.return_value = mock_app
+        
+        start_a2a_server(mock_agent_executor)
+        
+        mock_create_app.assert_called_once_with(
+            mock_agent_executor, 
+            transport="starlette"
+        )
+        
+        mock_uvicorn_run.assert_called_once_with(
+            mock_app, 
+            host="localhost", 
+            port=10001
+        )
+    
+    @patch('uvicorn.run')
+    @patch('crewai.a2a.server.create_a2a_app')
+    def test_start_a2a_server_custom(self, mock_create_app, mock_uvicorn_run, mock_agent_executor):
+        """Test starting A2A server with custom parameters."""
+        mock_app = Mock()
+        mock_create_app.return_value = mock_app
+        
+        start_a2a_server(
+            mock_agent_executor,
+            host="0.0.0.0",
+            port=8080,
+            transport="fastapi"
+        )
+        
+        mock_create_app.assert_called_once_with(
+            mock_agent_executor,
+            transport="fastapi"
+        )
+        
+        mock_uvicorn_run.assert_called_once_with(
+            mock_app,
+            host="0.0.0.0",
+            port=8080
+        )
+    
+    @patch('crewai.a2a.server.A2AStarletteApplication')
+    @patch('crewai.a2a.server.DefaultRequestHandler')
+    @patch('crewai.a2a.server.InMemoryTaskStore')
+    def test_create_a2a_app_starlette(self, mock_task_store_class, mock_handler_class, mock_app_class, mock_agent_executor):
+        """Test creating A2A app with Starlette transport."""
+        mock_handler = Mock()
+        mock_app_instance = Mock()
+        mock_built_app = Mock()
+        mock_task_store = Mock()
+        
+        mock_task_store_class.return_value = mock_task_store
+        mock_handler_class.return_value = mock_handler
+        mock_app_class.return_value = mock_app_instance
+        mock_app_instance.build.return_value = mock_built_app
+        
+        result = create_a2a_app(mock_agent_executor, transport="starlette")
+        
+        mock_task_store_class.assert_called_once()
+        mock_handler_class.assert_called_once_with(mock_agent_executor, mock_task_store)
+        mock_app_class.assert_called_once()
+        mock_app_instance.build.assert_called_once()
+        
+        assert result == mock_built_app
+    
+    def test_create_a2a_app_fastapi(self, mock_agent_executor):
+        """Test creating A2A app with FastAPI transport raises error."""
+        with pytest.raises(ValueError, match="FastAPI transport is not available"):
+            create_a2a_app(
+                mock_agent_executor,
+                transport="fastapi",
+                agent_name="Custom Agent",
+                agent_description="Custom description"
+            )
+    
+    @patch('crewai.a2a.server.A2AStarletteApplication')
+    @patch('crewai.a2a.server.DefaultRequestHandler')
+    @patch('crewai.a2a.server.InMemoryTaskStore')
+    def test_create_a2a_app_default_transport(self, mock_task_store_class, mock_handler_class, mock_app_class, mock_agent_executor):
+        """Test creating A2A app with default transport."""
+        mock_handler = Mock()
+        mock_app_instance = Mock()
+        mock_built_app = Mock()
+        mock_task_store = Mock()
+        
+        mock_task_store_class.return_value = mock_task_store
+        mock_handler_class.return_value = mock_handler
+        mock_app_class.return_value = mock_app_instance
+        mock_app_instance.build.return_value = mock_built_app
+        
+        result = create_a2a_app(mock_agent_executor)
+        
+        mock_task_store_class.assert_called_once()
+        mock_handler_class.assert_called_once_with(mock_agent_executor, mock_task_store)
+        mock_app_class.assert_called_once()
+        assert result == mock_built_app
+
+
+@pytest.mark.skipif(A2A_AVAILABLE, reason="Testing import error handling")
+def test_server_import_error_handling():
+    """Test that import errors are handled gracefully when A2A is not available."""
+    with pytest.raises(ImportError, match="A2A integration requires"):
+        from crewai.a2a.server import start_a2a_server