feat: ability to retrieve agents session, turn, step by ids (#1286)

# What does this PR do?

- Fix up rotten implementation for retrieving agent's Session, Turn,
Step with actual working implementation.

- Update `getting_started` notebook with retrieving by agent session_id.
https://github.com/meta-llama/llama-stack/blob/export_agent_dataset/docs/getting_started.ipynb

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

## Test Plan

Test with script:
https://gist.github.com/yanxi0830/657cecee8f1f0e39d322963d9c0f598e

<img width="503" alt="image"
src="https://github.com/user-attachments/assets/5ea9bc33-83d1-40bc-98e1-b68393158387"
/>


[//]: # (## Documentation)

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

@@ -194,17 +194,13 @@ class MetaReferenceAgentsImpl(Agents):
             yield event
 
     async def get_agents_turn(self, agent_id: str, session_id: str, turn_id: str) -> Turn:
-        turn = await self.persistence_store.get(f"session:{agent_id}:{session_id}:{turn_id}")
-        turn = json.loads(turn)
-        turn = Turn(**turn)
+        agent = await self.get_agent(agent_id)
+        turn = await agent.storage.get_session_turn(session_id, turn_id)
         return turn
 
     async def get_agents_step(self, agent_id: str, session_id: str, turn_id: str, step_id: str) -> AgentStepResponse:
-        turn = await self.persistence_store.get(f"session:{agent_id}:{session_id}:{turn_id}")
-        turn = json.loads(turn)
-        turn = Turn(**turn)
-        steps = turn.steps
-        for step in steps:
+        turn = await self.get_agents_turn(agent_id, session_id, turn_id)
+        for step in turn.steps:
             if step.step_id == step_id:
                 return AgentStepResponse(step=step)
         raise ValueError(f"Provided step_id {step_id} could not be found")
@@ -215,20 +211,18 @@ class MetaReferenceAgentsImpl(Agents):
         session_id: str,
         turn_ids: Optional[List[str]] = None,
     ) -> Session:
-        session = await self.persistence_store.get(f"session:{agent_id}:{session_id}")
-        session = Session(**json.loads(session), turns=[])
-        turns = []
+        agent = await self.get_agent(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")
+        turns = await agent.storage.get_session_turns(session_id)
         if turn_ids:
-            for turn_id in turn_ids:
-                turn = await self.persistence_store.get(f"session:{agent_id}:{session_id}:{turn_id}")
-                turn = json.loads(turn)
-                turn = Turn(**turn)
-                turns.append(turn)
+            turns = [turn for turn in turns if turn.turn_id in turn_ids]
         return Session(
-            session_name=session.session_name,
+            session_name=session_info.session_name,
             session_id=session_id,
-            turns=turns if turns else [],
-            started_at=session.started_at,
+            turns=turns,
+            started_at=session_info.started_at,
         )
 
     async def delete_agents_session(self, agent_id: str, session_id: str) -> None: