Post a message whose attachment can't be previewed instead of failing

The thumbnail or video preview is generated while the message is posted.
When ffmpeg or libvips can't decode the file (a truncated upload, an .mp4
holding only audio), the error escaped after the message had been saved:
the request failed with a 500, the message was never broadcast, and the
sender's upload stayed at 100% while the rest of the files in that drop
were never sent. Posting the message without a preview keeps the file and
lets everyone see it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JVFo3Lt9T8M5NR7KxVvsZ2

test/controllers/messages_controller_test.rb

@@ -57,6 +57,18 @@ class MessagesControllerTest < ActionDispatch::IntegrationTest
     end
   end
 
+  test "creating a message with an image that can't be decoded broadcasts the message to the room" do
+    webp = Vips::Image.new_from_file(file_fixture("moon.jpg").to_s).webpsave_buffer
+    broken = Rack::Test::UploadedFile.new(StringIO.new(webp.byteslice(0, webp.bytesize / 2)), "image/webp", original_filename: "broken.webp")
+
+    post room_messages_url(@room, format: :turbo_stream), params: { message: { attachment: broken, client_message_id: 999 } }
+
+    assert_response :success
+    assert_rendered_turbo_stream_broadcast @room, :messages, action: "append", target: [ @room, :messages ] do
+      assert_select ".message__body a[href*='broken.webp']"
+    end
+  end
+
   test "creating a message broadcasts unread room to each member" do
     @room.users.each do |member|
       assert_broadcasts UnreadRoomsChannel.stream_name_for(member.id), 1 do