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/persistence.py

@@ -21,6 +21,7 @@ log = logging.getLogger(__name__)
 class AgentSessionInfo(BaseModel):
     session_id: str
     session_name: str
+    # TODO: is this used anywhere?
     vector_db_id: Optional[str] = None
     started_at: datetime
 
@@ -85,6 +86,14 @@ class AgentPersistence:
         turns.sort(key=lambda x: (x.completed_at or datetime.min))
         return turns
 
+    async def get_session_turn(self, session_id: str, turn_id: str) -> Optional[Turn]:
+        value = await self.kvstore.get(
+            key=f"session:{self.agent_id}:{session_id}:{turn_id}",
+        )
+        if not value:
+            return None
+        return Turn(**json.loads(value))
+
     async def set_in_progress_tool_call_step(self, session_id: str, turn_id: str, step: ToolExecutionStep):
         await self.kvstore.set(
             key=f"in_progress_tool_call_step:{self.agent_id}:{session_id}:{turn_id}",