feat: make agent_id optional in conversations list endpoint [LET-7612] (#9585)

* feat: make agent_id optional in conversations list endpoint [LET-7612]

Allow listing all conversations without filtering by agent_id.

**Router changes (conversations.py):**
- Changed agent_id from required (`...`) to optional (`None`)
- Updated description to clarify behavior
- Updated docstring to reflect optional filtering

**Manager changes (conversation_manager.py):**
- Updated list_conversations signature: agent_id: str → Optional[str]
- Updated docstring to clarify optional behavior
- Summary search query: conditionally adds agent_id filter only if provided
- Default list logic: passes agent_id (can be None) to list_async

**How it works:**
- Without agent_id: returns all conversations for the user's organization
- With agent_id: returns conversations filtered by that agent
- list_async handles None gracefully via **kwargs pattern

**Use case:**
- Cloud UI can list all user conversations across agents
- Still supports filtering by agent_id when needed

🐾 Generated with [Letta Code](https://letta.com)

Co-Authored-By: Letta <noreply@letta.com>

* chore: update logs

* chore: update logs

---------

Co-authored-by: Letta <noreply@letta.com>

letta/server/rest_api/routers/v1/conversations.py

@@ -60,14 +60,14 @@ async def create_conversation(
 
 @router.get("/", response_model=List[Conversation], operation_id="list_conversations")
 async def list_conversations(
-    agent_id: str = Query(..., description="The agent ID to list conversations for"),
+    agent_id: Optional[str] = Query(None, description="The agent ID to list conversations for (optional - returns all conversations if not provided)"),
     limit: int = Query(50, description="Maximum number of conversations to return"),
     after: Optional[str] = Query(None, description="Cursor for pagination (conversation ID)"),
     summary_search: Optional[str] = Query(None, description="Search for text within conversation summaries"),
     server: SyncServer = Depends(get_letta_server),
     headers: HeaderParams = Depends(get_headers),
 ):
-    """List all conversations for an agent."""
+    """List all conversations for an agent (or all conversations if agent_id not provided)."""
     actor = await server.user_manager.get_actor_or_default_async(actor_id=headers.actor_id)
     return await conversation_manager.list_conversations(
         agent_id=agent_id,