Fix: RedisSessionService.list_sessions() state loss bug

## Problem
RedisSessionService.list_sessions() was clearing session state by setting
`session.state = {}` at line 178, causing loss of app-level and user-level
state data when listing sessions.

This differed from InMemorySessionService.list_sessions() which correctly
preserves state by calling `self._merge_state()`.

## Impact
- Session state (including custom fields like 'title', '_ag_ui_thread_id')
  was lost when listing sessions via the API
- Frontend applications couldn't display session metadata in session lists
- State was only available when fetching individual sessions via get_session()

## Root Cause
The list_sessions() method was directly clearing state instead of merging
app/user state from Redis hashes, breaking the state management contract.

## Solution
Replace `session.state = {}` with `session = await self._merge_state(app_name, user_id, session)`
to match the behavior of InMemorySessionService.list_sessions()

Author: BaskaranNM

src/google/adk_community/sessions/redis_session_service.py

@@ -175,7 +175,7 @@ async def list_sessions(
         for session_data in sessions.values():
             session = Session.model_validate(session_data)
             session.events = []
-            session.state = {}
+            session = await self._merge_state(app_name, user_id, session)
             sessions_without_events.append(session)
 
         return ListSessionsResponse(sessions=sessions_without_events)