fix(sdk): correct WebSocket URL pattern for platform-agent

AgentApiWrapper._build_ws_url was building /v1/agent_ws/{chat_id}/
which does not exist in platform-agent. Fixed to /agent_ws/?thread_id={chat_id}
to match the actual endpoint and query-param isolation scheme.

Also simplify Matrix MVP settings handlers to MVP_UNAVAILABLE stubs
and add handle_unknown_command for unregistered !commands.

tests/platform/test_real.py

@@ -159,7 +159,7 @@ def test_agent_api_wrapper_falls_back_to_legacy_url_constructor(monkeypatch):
 
     wrapper = AgentApiWrapper(
         agent_id="agent-2",
-        url="https://agent.example.com/v1/agent_ws/chat-9/",
+        url="https://agent.example.com/agent_ws/",
         chat_id="chat-9",
         callback="cb",
     )
@@ -167,7 +167,7 @@ def test_agent_api_wrapper_falls_back_to_legacy_url_constructor(monkeypatch):
     assert calls == [
         {
             "agent_id": "agent-2",
-            "url": "https://agent.example.com/v1/agent_ws/chat-9/",
+            "url": "https://agent.example.com/agent_ws/?thread_id=chat-9",
             "callback": "cb",
             "on_disconnect": None,
         }