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>

fern/openapi.json

@@ -8718,19 +8718,26 @@
       "get": {
         "tags": ["conversations"],
         "summary": "List Conversations",
-        "description": "List all conversations for an agent.",
+        "description": "List all conversations for an agent (or all conversations if agent_id not provided).",
         "operationId": "list_conversations",
         "parameters": [
           {
             "name": "agent_id",
             "in": "query",
-            "required": true,
+            "required": false,
             "schema": {
-              "type": "string",
-              "description": "The agent ID to list conversations for",
+              "anyOf": [
+                {
+                  "type": "string"
+                },
+                {
+                  "type": "null"
+                }
+              ],
+              "description": "The agent ID to list conversations for (optional - returns all conversations if not provided)",
               "title": "Agent Id"
             },
-            "description": "The agent ID to list conversations for"
+            "description": "The agent ID to list conversations for (optional - returns all conversations if not provided)"
           },
           {
             "name": "limit",