FIX: CreateThread allows thread creation in closed/read-only/archived channels (#37661)

## Summary

`Chat::CreateThread` allows thread creation in closed, read_only, and
archived channels because it only checks `can_preview_chat_channel?`
(view access) without verifying the channel's status permits content
creation.

## Source

- Patch Triage: https://patch.discourse.org/patch-triage/237
- Original Commit:
https://github.com/discourse/discourse/blob/main/plugins/chat/app/controllers/chat/api/channel_threads_controller.rb

---

🤖 Generated via [Patch Triage](https://patch.discourse.org/patch-triage)

plugins/chat/app/services/chat/create_thread.rb

@@ -28,6 +28,7 @@ module Chat
 
     model :channel
     policy :can_view_channel
+    policy :can_create_thread_in_channel
     policy :threading_enabled_for_channel
     model :original_message
 
@@ -49,6 +50,10 @@ module Chat
       guardian.can_preview_chat_channel?(channel)
     end
 
+    def can_create_thread_in_channel(guardian:, channel:)
+      guardian.can_create_channel_message?(channel)
+    end
+
     def threading_enabled_for_channel(channel:)
       channel.threading_enabled?
     end

plugins/chat/spec/services/chat/create_thread_spec.rb

@@ -106,6 +106,32 @@ RSpec.describe Chat::CreateThread do
       it { is_expected.to fail_a_policy(:can_view_channel) }
     end
 
+    context "when channel is not open" do
+      context "when channel is read_only" do
+        before { channel_1.update!(status: :read_only) }
+
+        it { is_expected.to fail_a_policy(:can_create_thread_in_channel) }
+      end
+
+      context "when channel is closed" do
+        before { channel_1.update!(status: :closed) }
+
+        it { is_expected.to fail_a_policy(:can_create_thread_in_channel) }
+
+        context "when user is staff" do
+          let(:guardian) { Guardian.new(Fabricate(:admin)) }
+
+          it { is_expected.to run_successfully }
+        end
+      end
+
+      context "when channel is archived" do
+        before { channel_1.update!(status: :archived) }
+
+        it { is_expected.to fail_a_policy(:can_create_thread_in_channel) }
+      end
+    end
+
     context "when threading is not enabled for the channel" do
       before { channel_1.update!(threading_enabled: false) }