Fix remaining 12 multi-line conn shadowing issues

Manually refactor multi-line conn = post/put(conn, ...) patterns
across user_session, user_settings, and user_registration controller
tests. Rename shadowed conn to response using pipeline operator.

Also add plans/multi-line-conn-refactoring.md spec for future
Elixir-based tooling to handle these patterns automatically.

app/test/firehose_web/controllers/user_session_controller_test.exs

@@ -73,20 +73,22 @@ defmodule FirehoseWeb.UserSessionControllerTest do
     test "logs the user in", %{conn: conn, user: user} do
       user = set_password(user)
 
-      conn =
-        post(conn, ~p"/users/log-in", %{
+      response =
+        conn
+        |> post(~p"/users/log-in", %{
           "user" => %{"email" => user.email, "password" => valid_user_password()}
         })
 
-      assert get_session(conn, :user_token)
-      assert redirected_to(conn) == ~p"/"
+      assert get_session(response, :user_token)
+      assert redirected_to(response) == ~p"/"
     end
 
     test "logs the user in with remember me", %{conn: conn, user: user} do
       user = set_password(user)
 
-      conn =
-        post(conn, ~p"/users/log-in", %{
+      response =
+        conn
+        |> post(~p"/users/log-in", %{
           "user" => %{
             "email" => user.email,
             "password" => valid_user_password(),
@@ -94,14 +96,14 @@ defmodule FirehoseWeb.UserSessionControllerTest do
           }
         })
 
-      assert conn.resp_cookies["_firehose_web_user_remember_me"]
-      assert redirected_to(conn) == ~p"/"
+      assert response.resp_cookies["_firehose_web_user_remember_me"]
+      assert redirected_to(response) == ~p"/"
     end
 
     test "logs the user in with return to", %{conn: conn, user: user} do
       user = set_password(user)
 
-      conn =
+      response =
         conn
         |> init_test_session(user_return_to: "/foo/bar")
         |> post(~p"/users/log-in", %{
@@ -111,17 +113,18 @@ defmodule FirehoseWeb.UserSessionControllerTest do
           }
         })
 
-      assert redirected_to(conn) == "/foo/bar"
-      assert Phoenix.Flash.get(conn.assigns.flash, :info) =~ "Welcome back!"
+      assert redirected_to(response) == "/foo/bar"
+      assert Phoenix.Flash.get(response.assigns.flash, :info) =~ "Welcome back!"
     end
 
     test "emits error message with invalid credentials", %{conn: conn, user: user} do
-      conn =
-        post(conn, ~p"/users/log-in?mode=password", %{
+      response =
+        conn
+        |> post(~p"/users/log-in?mode=password", %{
           "user" => %{"email" => user.email, "password" => "invalid_password"}
         })
+        |> html_response(200)
 
-      response = html_response(conn, 200)
       assert response =~ "Log in"
       assert response =~ "Invalid email or password"
     end
@@ -129,51 +132,56 @@ defmodule FirehoseWeb.UserSessionControllerTest do
 
   describe "POST /users/log-in - magic link" do
     test "sends magic link email when user exists", %{conn: conn, user: user} do
-      conn =
-        post(conn, ~p"/users/log-in", %{
+      response =
+        conn
+        |> post(~p"/users/log-in", %{
           "user" => %{"email" => user.email}
         })
 
-      assert Phoenix.Flash.get(conn.assigns.flash, :info) =~ "If your email is in our system"
+      assert Phoenix.Flash.get(response.assigns.flash, :info) =~ "If your email is in our system"
       assert Firehose.Repo.get_by!(Accounts.UserToken, user_id: user.id).context == "login"
     end
 
     test "logs the user in", %{conn: conn, user: user} do
       {token, _hashed_token} = generate_user_magic_link_token(user)
 
-      conn =
-        post(conn, ~p"/users/log-in", %{
+      response =
+        conn
+        |> post(~p"/users/log-in", %{
           "user" => %{"token" => token}
         })
 
-      assert get_session(conn, :user_token)
-      assert redirected_to(conn) == ~p"/"
+      assert get_session(response, :user_token)
+      assert redirected_to(response) == ~p"/"
     end
 
     test "confirms unconfirmed user", %{conn: conn, unconfirmed_user: user} do
       {token, _hashed_token} = generate_user_magic_link_token(user)
       refute user.confirmed_at
 
-      conn =
-        post(conn, ~p"/users/log-in", %{
+      response =
+        conn
+        |> post(~p"/users/log-in", %{
           "user" => %{"token" => token},
           "_action" => "confirmed"
         })
 
-      assert get_session(conn, :user_token)
-      assert redirected_to(conn) == ~p"/"
-      assert Phoenix.Flash.get(conn.assigns.flash, :info) =~ "User confirmed successfully."
+      assert get_session(response, :user_token)
+      assert redirected_to(response) == ~p"/"
+      assert Phoenix.Flash.get(response.assigns.flash, :info) =~ "User confirmed successfully."
 
       assert Accounts.get_user!(user.id).confirmed_at
     end
 
     test "emits error message when magic link is invalid", %{conn: conn} do
-      conn =
-        post(conn, ~p"/users/log-in", %{
+      response =
+        conn
+        |> post(~p"/users/log-in", %{
           "user" => %{"token" => "invalid"}
         })
+        |> html_response(200)
 
-      assert html_response(conn, 200) =~ "The link is invalid or it has expired."
+      assert response =~ "The link is invalid or it has expired."
     end
   end