A2A with agent engine session service in api_server runtime

**Describe the bug**
It's not possible to use agent-engine session service with A2A

**To Reproduce**
Run your ADK agent like this
`adk api_server --a2a --host 0.0.0.0 --port 8080 --session_service_uri "agentengine://${AGENT_ENGINE_ID}" --memory_service_uri "agentengine://${AGENT_ENGINE_ID}"`

You get this error when you use the A2A Endpoint (I use it with Agent Space (a.k.a Gemini Enterprise))
```
Traceback (most recent call last):
File "/usr/local/lib/python3.12/site-packages/google/adk/a2a/executor/a2a_agent_executor.py", line 167, in execute
await self._handle_request(context, event_queue)
File "/usr/local/lib/python3.12/site-packages/google/adk/a2a/executor/a2a_agent_executor.py", line 208, in _handle_request
session = await self._prepare_session(context, run_request, runner)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.12/site-packages/google/adk/a2a/executor/a2a_agent_executor.py", line 303, in _prepare_session
session = await runner.session_service.get_session(
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.12/site-packages/google/adk/sessions/vertex_ai_session_service.py", line 162, in get_session
get_session_response, events_iterator = await asyncio.gather(
^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.12/site-packages/vertexai/_genai/sessions.py", line 882, in get
response = await self._api_client.async_request(
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.12/site-packages/google/genai/_api_client.py", line 1365, in async_request
result = await self._async_request(
^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.12/site-packages/google/genai/_api_client.py", line 1310, in _async_request
return await self._async_retry( # type: ignore[no-any-return]
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.12/site-packages/tenacity/asyncio/__init__.py", line 111, in __call__
do = await self.iter(retry_state=retry_state)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.12/site-packages/tenacity/asyncio/__init__.py", line 153, in iter
result = await action(retry_state)
^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.12/site-packages/tenacity/_utils.py", line 99, in inner
return call(*args, **kwargs)
^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.12/site-packages/tenacity/__init__.py", line 420, in exc_check
raise retry_exc.reraise()
^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.12/site-packages/tenacity/__init__.py", line 187, in reraise
raise self.last_attempt.result()
^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.12/concurrent/futures/_base.py", line 449, in result
return self.__get_result()
^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.12/concurrent/futures/_base.py", line 401, in __get_result
raise self._exception
File "/usr/local/lib/python3.12/site-packages/tenacity/asyncio/__init__.py", line 114, in __call__
result = await fn(*args, **kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.12/site-packages/google/genai/_api_client.py", line 1290, in _async_request_once
await errors.APIError.raise_for_async_response(client_response)
File "/usr/local/lib/python3.12/site-packages/google/genai/errors.py", line 166, in raise_for_async_response
raise ClientError(status_code, response_json, response)
google.genai.errors.ClientError: 400 INVALID_ARGUMENT. {'error': {'code': 400, 'message': 'List of found errors:\t1.Field: name; Message: Invalid Session resource name.\t', 'status': 'INVALID_ARGUMENT', 'details': [{'@type': 'type.googleapis.com/google.rpc.BadRequest', 'fieldViolations': [{'field': 'name', 'description': 'Invalid Session resource name.'}]}]}}
```

If you start the agent without the session and memory service
`adk api_server --a2a --host 0.0.0.0 --port 8080`

You will use the InMemoryService and it works well

**Expected behavior**
I expect no error when I use AgentEngine memory service and A2A.

A2A invocation belonging to an external process/flow/session management, I would assume that the A2A request do not need persistent session service. So, having InMemory session service only for the A2A endpoint is fine in my opinion

**Desktop (please complete the following information):**
- Python version(python -V): 3.12.11
- ADK version(pip show google-adk): 1.18.0

**Model Information:**
- Are you using LiteLLM: No
- Which model is being used(e.g. gemini-2.5-pro): gemini-2.5-flash

**Additional context**
I would imagine solving the issue [here](https://github.com/google/adk-python/blob/0b1784e0e493a0e2df1edfe37e5ed5f4247e7d9d/src/google/adk/cli/fast_api.py#L347)

And I imagine a runner update like this.
```
async def _get_a2a_runner_async() -> Runner:
original_runner = await adk_web_server.get_runner_async(captured_app_name)
runner = copy.copy(original_runner) # Create a shallow copy
runner.memory_service = InMemoryMemoryService()
runner.session_service = InMemorySessionService()
runner.artifact_service = InMemoryArtifactService()
runner.credential_service = InMemoryCredentialService()
return runner

return _get_a2a_runner_async
```

What do you think?