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/agent_instance.py

@@ -61,7 +61,12 @@ from llama_stack.apis.inference import (
     UserMessage,
 )
 from llama_stack.apis.safety import Safety
-from llama_stack.apis.tools import RAGDocument, ToolGroups, ToolInvocationResult, ToolRuntime
+from llama_stack.apis.tools import (
+    RAGDocument,
+    ToolGroups,
+    ToolInvocationResult,
+    ToolRuntime,
+)
 from llama_stack.apis.vector_io import VectorIO
 from llama_stack.models.llama.datatypes import (
     BuiltinTool,