fix: fix llama stack run with missing agent impl (#1559)

# What does this PR do?

- recent merge https://github.com/meta-llama/llama-stack/pull/1410
introduce error
```
ValueError: Provider meta-reference (Api.agents) does not implement the following methods:
[('list_agent_sessions', 'not_actually_implemented'), ('list_agents', 'not_actually_implemented')]
```

[//]: # (If resolving an issue, uncomment and update the line below)
[//]: # (Closes #[issue-number])

## Test Plan
```
llama stack run
```

```
LLAMA_STACK_CONFIG=fireworks pytest -v tests/integration/agents/test_agents.py --text-model meta-llama/Llama-3.3-70B-Instruct
```

1379530386

[//]: # (## Documentation)

llama_stack/providers/inline/agents/meta_reference/agents.py

@@ -12,6 +12,7 @@ import uuid
 from typing import AsyncGenerator, List, Optional, Union
 
 from llama_stack.apis.agents import (
+    Agent,
     AgentConfig,
     AgentCreateResponse,
     Agents,
@@ -21,6 +22,8 @@ from llama_stack.apis.agents import (
     AgentTurnCreateRequest,
     AgentTurnResumeRequest,
     Document,
+    ListAgentSessionsResponse,
+    ListAgentsResponse,
     Session,
     Turn,
 )
@@ -84,7 +87,7 @@ class MetaReferenceAgentsImpl(Agents):
             agent_id=agent_id,
         )
 
-    async def get_agent(self, agent_id: str) -> ChatAgent:
+    async def _get_agent_impl(self, agent_id: str) -> ChatAgent:
         agent_config = await self.persistence_store.get(
             key=f"agent:{agent_id}",
         )
@@ -120,7 +123,7 @@ class MetaReferenceAgentsImpl(Agents):
         agent_id: str,
         session_name: str,
     ) -> AgentSessionCreateResponse:
-        agent = await self.get_agent(agent_id)
+        agent = await self._get_agent_impl(agent_id)
 
         session_id = await agent.create_session(session_name)
         return AgentSessionCreateResponse(
@@ -160,7 +163,7 @@ class MetaReferenceAgentsImpl(Agents):
         self,
         request: AgentTurnCreateRequest,
     ) -> AsyncGenerator:
-        agent = await self.get_agent(request.agent_id)
+        agent = await self._get_agent_impl(request.agent_id)
         async for event in agent.create_and_execute_turn(request):
             yield event
 
@@ -188,12 +191,12 @@ class MetaReferenceAgentsImpl(Agents):
         self,
         request: AgentTurnResumeRequest,
     ) -> AsyncGenerator:
-        agent = await self.get_agent(request.agent_id)
+        agent = await self._get_agent_impl(request.agent_id)
         async for event in agent.resume_turn(request):
             yield event
 
     async def get_agents_turn(self, agent_id: str, session_id: str, turn_id: str) -> Turn:
-        agent = await self.get_agent(agent_id)
+        agent = await self._get_agent_impl(agent_id)
         turn = await agent.storage.get_session_turn(session_id, turn_id)
         return turn
 
@@ -210,7 +213,7 @@ class MetaReferenceAgentsImpl(Agents):
         session_id: str,
         turn_ids: Optional[List[str]] = None,
     ) -> Session:
-        agent = await self.get_agent(agent_id)
+        agent = await self._get_agent_impl(agent_id)
         session_info = await agent.storage.get_session_info(session_id)
         if session_info is None:
             raise ValueError(f"Session {session_id} not found")
@@ -232,3 +235,15 @@ class MetaReferenceAgentsImpl(Agents):
 
     async def shutdown(self) -> None:
         pass
+
+    async def list_agents(self) -> ListAgentsResponse:
+        pass
+
+    async def get_agent(self, agent_id: str) -> Agent:
+        pass
+
+    async def list_agent_sessions(
+        self,
+        agent_id: str,
+    ) -> ListAgentSessionsResponse:
+        pass