feat: Add an to_a2a util to convert adk agent to A2A ASGI application

Users can do :
```
a2a_app = to_a2a(root_agent)
```
then use below command to start a2a server:
```
uvicorn user_module:a2a_app --host localhost --port 8000
```

PiperOrigin-RevId: 785625048

src/google/adk/a2a/utils/agent_to_a2a.py

@@ -0,0 +1,118 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from __future__ import annotations
+
+import logging
+import sys
+
+try:
+  from a2a.server.apps import A2AStarletteApplication
+  from a2a.server.request_handlers import DefaultRequestHandler
+  from a2a.server.tasks import InMemoryTaskStore
+except ImportError as e:
+  if sys.version_info < (3, 10):
+    raise ImportError(
+        "A2A requires Python 3.10 or above. Please upgrade your Python version."
+    ) from e
+  else:
+    raise e
+
+from starlette.applications import Starlette
+
+from ...agents.base_agent import BaseAgent
+from ...artifacts.in_memory_artifact_service import InMemoryArtifactService
+from ...auth.credential_service.in_memory_credential_service import InMemoryCredentialService
+from ...cli.utils.logs import setup_adk_logger
+from ...memory.in_memory_memory_service import InMemoryMemoryService
+from ...runners import Runner
+from ...sessions.in_memory_session_service import InMemorySessionService
+from ..executor.a2a_agent_executor import A2aAgentExecutor
+from .agent_card_builder import AgentCardBuilder
+
+
+def to_a2a(
+    agent: BaseAgent, *, host: str = "localhost", port: int = 8000
+) -> Starlette:
+  """Convert an ADK agent to a A2A Starlette application.
+
+  Args:
+      agent: The ADK agent to convert
+      host: The host for the A2A RPC URL (default: "localhost")
+      port: The port for the A2A RPC URL (default: 8000)
+
+  Returns:
+      A Starlette application that can be run with uvicorn
+
+  Example:
+      agent = MyAgent()
+      app = to_a2a(agent, host="localhost", port=8000)
+      # Then run with: uvicorn module:app --host localhost --port 8000
+  """
+  # Set up ADK logging to ensure logs are visible when using uvicorn directly
+  setup_adk_logger(logging.INFO)
+
+  async def create_runner() -> Runner:
+    """Create a runner for the agent."""
+    return Runner(
+        app_name=agent.name or "adk_agent",
+        agent=agent,
+        # Use minimal services - in a real implementation these could be configured
+        artifact_service=InMemoryArtifactService(),
+        session_service=InMemorySessionService(),
+        memory_service=InMemoryMemoryService(),
+        credential_service=InMemoryCredentialService(),
+    )
+
+  # Create A2A components
+  task_store = InMemoryTaskStore()
+
+  agent_executor = A2aAgentExecutor(
+      runner=create_runner,
+  )
+
+  request_handler = DefaultRequestHandler(
+      agent_executor=agent_executor, task_store=task_store
+  )
+
+  # Build agent card
+  rpc_url = f"http://{host}:{port}/"
+  card_builder = AgentCardBuilder(
+      agent=agent,
+      rpc_url=rpc_url,
+  )
+
+  # Create a Starlette app that will be configured during startup
+  app = Starlette()
+
+  # Add startup handler to build the agent card and configure A2A routes
+  async def setup_a2a():
+    # Build the agent card asynchronously
+    agent_card = await card_builder.build()
+
+    # Create the A2A Starlette application
+    a2a_app = A2AStarletteApplication(
+        agent_card=agent_card,
+        http_handler=request_handler,
+    )
+
+    # Add A2A routes to the main app
+    a2a_app.add_routes_to_app(
+        app,
+    )
+
+  # Store the setup function to be called during startup
+  app.add_event_handler("startup", setup_a2a)
+
+  return app