fix(coderd): reject pinning child chats in patchChat handler (#24669)

The UI already prevents child (delegated/subagent) chats from being
pinned, but the `PATCH /api/experimental/chats/{chat}` endpoint did not
enforce this. A direct API call could pin a child chat.

- Add a `400 Bad Request` guard in `patchChat` when `pinOrder > 0` and
the chat has a `ParentChatID`
- Add `TestChatPinOrder/RejectsChildChat` test

> 🤖

coderd/database/dump.sql

@@ -1483,7 +1483,9 @@ CREATE TABLE chats (
     dynamic_tools jsonb,
     organization_id uuid NOT NULL,
     plan_mode chat_plan_mode,
-    client_type chat_client_type DEFAULT 'api'::chat_client_type NOT NULL
+    client_type chat_client_type DEFAULT 'api'::chat_client_type NOT NULL,
+    CONSTRAINT chats_pin_order_archived_check CHECK (((pin_order = 0) OR (archived = false))),
+    CONSTRAINT chats_pin_order_parent_check CHECK (((pin_order = 0) OR (parent_chat_id IS NULL)))
 );
 
 CREATE TABLE connection_logs (