fix: Fix list_sessions() (#3114)

# What does this PR do?
1. Updates `AgentPersistence.list_sessions()` to properly filter out
`Turn` keys from `Session` keys.
2. Adds a suite of unit tests to confirm the `list_sessions()` behavior
and tests the failed sample in
https://github.com/meta-llama/llama-stack/issues/3048

## Fixes https://github.com/meta-llama/llama-stack/issues/3048


## Test Plan
Unit tests added.

---------

Signed-off-by: Francisco Javier Arceo <farceo@redhat.com>

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

@@ -191,7 +191,11 @@ class AgentPersistence:
         sessions = []
         for value in values:
             try:
-                session_info = Session(**json.loads(value))
+                data = json.loads(value)
+                if "turn_id" in data:
+                    continue
+
+                session_info = Session(**data)
                 sessions.append(session_info)
             except Exception as e:
                 log.error(f"Error parsing session info: {e}")