DEV: chat streaming (#25736)

This commit introduces the possibility to stream messages. To allow plugins to use streaming this commit also ships a `ChatSDK` library to allow to interact with few parts of discourse chat.

```ruby
ChatSDK::Message.create_with_stream(raw: "test") do |helper|
  5.times do |i|
    is_streaming = helper.stream(raw: "more #{i}")
    next if !is_streaming
    sleep 2
  end
end
```

This commit also introduces all the frontend parts:
- messages can now be marked as streaming
- when streaming their content will be updated when a new content is appended
- a special UI will be showing (a blinking indicator)
- a cancel button allows the user to stop the streaming, when cancelled `helper.stream(...)` will return `false`, and the plugin can decide exit early

plugins/chat/lib/chat_sdk/thread.rb

@@ -0,0 +1,80 @@
+# frozen_string_literal: true
+
+module ChatSDK
+  class Thread
+    include Chat::WithServiceHelper
+
+    # Updates the title of a specified chat thread.
+    #
+    # @param title [String] The new title for the chat thread.
+    # @param thread_id [Integer] The ID of the chat thread to be updated.
+    # @param guardian [Guardian] The guardian object representing the user's permissions.
+    # @return [Chat::Thread] The updated thread object with the new title.
+    #
+    # @example Updating the title of a chat thread
+    #   ChatSDK::Thread.update_title(title: "New Thread Title", thread_id: 1, guardian: Guardian.new)
+    def self.update_title(**params)
+      new.update(title: params[:title], thread_id: params[:thread_id], guardian: params[:guardian])
+    end
+
+    def self.update(**params)
+      new.update(**params)
+    end
+
+    # Retrieves messages from a specified thread.
+    #
+    # @param thread_id [Integer] The ID of the chat thread from which to fetch messages.
+    # @param guardian [Guardian] The guardian object representing the user's permissions.
+    # @return [Array<Chat::Message>] An array of message objects from the specified thread.
+    #
+    # @example Fetching messages from a thread with additional parameters
+    #   ChatSDK::Thread.messages(thread_id: 1, guardian: Guardian.new)
+    #
+    def self.messages(thread_id:, guardian:, **params)
+      new.messages(thread_id: thread_id, guardian: guardian, **params)
+    end
+
+    def messages(thread_id:, guardian:, **params)
+      with_service(
+        Chat::ListChannelThreadMessages,
+        thread_id: thread_id,
+        guardian: guardian,
+        **params,
+        direction: "future",
+      ) do
+        on_success { result.messages }
+        on_failed_policy(:can_view_thread) { raise "Guardian can't view thread" }
+        on_failed_policy(:target_message_exists) { raise "Target message doesn't exist" }
+        on_failed_policy(:ensure_thread_enabled) do
+          raise "Threading is not enabled for this channel"
+        end
+        on_failure do
+          p Chat::StepsInspector.new(result)
+          raise "Unexpected error"
+        end
+      end
+    end
+
+    def update(**params)
+      with_service(Chat::UpdateThread, **params) do
+        on_model_not_found(:channel) do
+          raise "Couldn’t find channel with id: `#{params[:channel_id]}`"
+        end
+        on_model_not_found(:thread) do
+          raise "Couldn’t find thread with id: `#{params[:thread_id]}`"
+        end
+        on_failed_policy(:can_view_channel) { raise "Guardian can't view channel" }
+        on_failed_policy(:can_edit_thread) { raise "Guardian can't edit thread" }
+        on_failed_policy(:threading_enabled_for_channel) do
+          raise "Threading is not enabled for this channel"
+        end
+        on_failed_contract { |contract| raise contract.errors.full_messages.join(", ") }
+        on_success { result.thread_instance }
+        on_failure do
+          p Chat::StepsInspector.new(result)
+          raise "Unexpected error"
+        end
+      end
+    end
+  end
+end