Fix Slack auth filter not halting request on invalid signature

`valid_slack_request?` returned a boolean but never called `head` or
`render`, so unauthenticated Slack requests were silently allowed through.
Added `head :unauthorized and return` to halt the filter chain, consistent
with the fix applied in #66.

Author: JuanVqz

app/controllers/slack/application_controller.rb

@@ -7,7 +7,7 @@ class Slack::ApplicationController < ApplicationController
   private
 
   def valid_slack_request?
-    @verified ||= verify_slack_signature
+    head :unauthorized and return unless verify_slack_signature
   end
 
   def verify_slack_signature

test/controllers/slack/puzzles_controller_test.rb

@@ -0,0 +1,56 @@
+require "test_helper"
+
+class Slack::PuzzlesControllerTest < ActionDispatch::IntegrationTest
+  setup do
+    ENV["SLACK_SIGNING_SECRET"] = "test_signing_secret"
+  end
+
+  test "rejects request with invalid Slack signature" do
+    post slack_puzzle_path, params: { payload: puzzle_payload },
+      headers: slack_headers(secret: "wrong_secret")
+
+    assert_response :unauthorized
+  end
+
+  test "rejects request with expired Slack timestamp" do
+    post slack_puzzle_path, params: { payload: puzzle_payload },
+      headers: slack_headers(timestamp: Time.now.to_i - 400)
+
+    assert_response :unauthorized
+  end
+
+  test "allows request with valid Slack signature" do
+    # Use a payload that fails model validation so no Slack API call is made,
+    # but the controller still renders 200 (its behavior on both save success/failure).
+    params = { payload: puzzle_payload(question: "") }
+
+    post slack_puzzle_path, params: params,
+      headers: slack_headers(body: params.to_query)
+
+    assert_response :ok
+  end
+
+  private
+
+  def puzzle_payload(question: "What is Ruby?")
+    {
+      user: { id: "U123" },
+      view: {
+        state: {
+          values: {
+            question: { question: { value: question } },
+            answer: { answer: { selected_option: { value: "ruby" } } },
+            explanation: { explanation: { value: "It is a programming language." } },
+            link: { link: { value: nil } }
+          }
+        }
+      }
+    }.to_json
+  end
+
+  def slack_headers(secret: ENV["SLACK_SIGNING_SECRET"], timestamp: Time.now.to_i, body: "")
+    ts = timestamp.to_s
+    sig = "v0=" + OpenSSL::HMAC.hexdigest("SHA256", secret, "v0:#{ts}:#{body}")
+    { "X-Slack-Request-Timestamp" => ts, "X-Slack-Signature" => sig }
+  end
+end