Remove unused DeveloperMessage model (#1207)

DeveloperMessage was a debug-only STI subclass of Message that was never
created by any production code. Remove the model, view partial, test,
and fixtures, and simplify Chat#conversation_messages accordingly.

https://claude.ai/code/session_012pm5HKGKFs1tpAsvXMr4Tp

Co-authored-by: Claude <noreply@anthropic.com>

app/models/chat.rb

@@ -75,10 +75,6 @@ class Chat < ApplicationRecord
   end
 
   def conversation_messages
-    if debug_mode?
-      messages
-    else
-      messages.where(type: [ "UserMessage", "AssistantMessage" ])
-    end
+    messages.where(type: [ "UserMessage", "AssistantMessage" ])
   end
 end

app/models/developer_message.rb

@@ -1,10 +0,0 @@
-class DeveloperMessage < Message
-  def role
-    "developer"
-  end
-
-  private
-    def broadcast?
-      chat.debug_mode?
-    end
-end

app/views/developer_messages/_developer_message.html.erb

@@ -1,6 +0,0 @@
-<%# locals: (developer_message:) %>
-
-<div id="<%= dom_id(developer_message) %>" class="my-2 <%= developer_message.debug? ? "bg-yellow-50 border-yellow-200" : "bg-blue-50 border-blue-200" %> px-3 py-2 rounded-lg max-w-[85%] ml-auto border">
-  <span class="text-secondary text-xs"><%= developer_message.debug? ? "Debug message (internal only)" : "System instruction (sent to AI)" %></span>
-  <p class="text-primary text-sm"><%= developer_message.content %></p>
-</div>

test/fixtures/messages.yml

@@ -1,17 +1,3 @@
-chat1_developer:
-  type: DeveloperMessage
-  content: You are a personal finance assistant.  Be concise and helpful.
-  chat: one
-  created_at: 2025-03-20 12:00:00
-  debug: false
-
-chat1_developer_debug:
-  type: DeveloperMessage
-  content: An internal debug message
-  chat: one
-  created_at: 2025-03-20 12:00:02
-  debug: true
-
 chat1_user:
   type: UserMessage
   content: Can you help me understand my spending habits?

test/models/developer_message_test.rb

@@ -1,28 +0,0 @@
-require "test_helper"
-
-class DeveloperMessageTest < ActiveSupport::TestCase
-  setup do
-    @chat = chats(:one)
-  end
-
-  test "does not broadcast" do
-    message = DeveloperMessage.create!(chat: @chat, content: "Some instructions")
-    message.update!(content: "updated")
-
-    assert_no_turbo_stream_broadcasts(@chat)
-  end
-
-  test "broadcasts if debug mode is enabled" do
-    with_env_overrides AI_DEBUG_MODE: "true" do
-      message = DeveloperMessage.create!(chat: @chat, content: "Some instructions")
-      message.update!(content: "updated")
-
-      streams = capture_turbo_stream_broadcasts(@chat)
-      assert_equal 2, streams.size
-      assert_equal "append", streams.first["action"]
-      assert_equal "messages", streams.first["target"]
-      assert_equal "update", streams.last["action"]
-      assert_equal "developer_message_#{message.id}", streams.last["target"]
-    end
-  end
-end