fix(messages): handle chat not found during message creation (#2896)

* fix(messages): handle chat not found during message creation

* test(messages): cover missing chat race

---------

Co-authored-by: sentry[bot] <39604003+sentry[bot]@users.noreply.github.com>
Co-authored-by: sure-admin <sure-admin@splashblot.com>

app/controllers/messages_controller.rb

@@ -15,6 +15,8 @@ class MessagesController < ApplicationController
     else
       redirect_to chat_path(@chat), alert: @message.errors.full_messages.to_sentence
     end
+  rescue ActiveRecord::InvalidForeignKey
+    redirect_to chats_path, alert: t(".chat_not_found")
   end
 
   # Called by the chat watchdog when an assistant "Thinking…" bubble has waited

config/locales/views/messages/en.yml

@@ -1,6 +1,8 @@
 ---
 en:
   messages:
+    create:
+      chat_not_found: This chat is no longer available.
     chat_form:
       placeholder: "Ask anything ..."
       disclaimer: "AI responses are informational only. Not financial advice!"

test/controllers/messages_controller_test.rb

@@ -12,6 +12,15 @@ class MessagesControllerTest < ActionDispatch::IntegrationTest
     assert_redirected_to chat_path(@chat, thinking: true)
   end
 
+  test "redirects to chats when message save races with chat deletion" do
+    UserMessage.any_instance.stubs(:save).raises(ActiveRecord::InvalidForeignKey)
+
+    post chat_messages_url(@chat), params: { message: { content: "Hello", ai_model: "gpt-4.1" } }
+
+    assert_redirected_to chats_path
+    assert_equal I18n.t("messages.create.chat_not_found"), flash[:alert]
+  end
+
   test "cannot create a message if AI is disabled" do
     @user.update!(ai_enabled: false)