feat(nginx): generate 502 config for apps without web listeners

When an app has no web listeners (not yet deployed, no web process type,
or web processes not running), generate a minimal nginx config that
returns 502 Bad Gateway instead of having no config at all. This ensures
domains resolve, monitoring tools detect non-200 status codes, and SSL
certificate provisioning tools like letsencrypt can function.

tests/unit/nginx-vhosts_13.bats

@@ -0,0 +1,98 @@
+#!/usr/bin/env bats
+
+load test_helper
+
+setup() {
+  global_setup
+  [[ -f "$DOKKU_ROOT/VHOST" ]] && cp -fp "$DOKKU_ROOT/VHOST" "$DOKKU_ROOT/VHOST.bak"
+}
+
+teardown() {
+  destroy_app
+  [[ -f "$DOKKU_ROOT/VHOST.bak" ]] && mv "$DOKKU_ROOT/VHOST.bak" "$DOKKU_ROOT/VHOST" && chown dokku:dokku "$DOKKU_ROOT/VHOST"
+  global_teardown
+}
+
+@test "(nginx-vhosts) proxy:build-config generates 502 config for undeployed app" {
+  run create_app
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+
+  run /bin/bash -c "test -f $DOKKU_ROOT/$TEST_APP/nginx.conf"
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+
+  run /bin/bash -c "dokku nginx:show-config $TEST_APP"
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+  assert_output_contains "return 502" -1
+}
+
+@test "(nginx-vhosts) proxy:build-config undeployed app returns 502" {
+  run create_app
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+
+  assert_http_localhost_response "http" "${TEST_APP}.${DOKKU_DOMAIN}" "80" "/" "" "502"
+}
+
+@test "(nginx-vhosts) proxy:build-config 502 config replaced after deploy" {
+  run deploy_app
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+
+  run /bin/bash -c "dokku nginx:show-config $TEST_APP"
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+  assert_output_contains "return 502" 0
+  assert_output_contains "proxy_pass" -1
+
+  assert_http_success "http://${TEST_APP}.${DOKKU_DOMAIN}"
+}
+
+@test "(nginx-vhosts) proxy:build-config domains:add updates 502 config" {
+  run create_app
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+
+  run /bin/bash -c "dokku domains:add $TEST_APP custom.${DOKKU_DOMAIN}"
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+
+  run /bin/bash -c "dokku nginx:show-config $TEST_APP"
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+  assert_output_contains "custom.${DOKKU_DOMAIN}" -1
+  assert_output_contains "return 502" -1
+}
+
+@test "(nginx-vhosts) proxy:build-config no nginx.conf when proxy disabled" {
+  run create_app
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+
+  run /bin/bash -c "test -f $DOKKU_ROOT/$TEST_APP/nginx.conf"
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+
+  run /bin/bash -c "dokku proxy:disable $TEST_APP"
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+
+  run /bin/bash -c "test -f $DOKKU_ROOT/$TEST_APP/nginx.conf"
+  echo "output: $output"
+  echo "status: $status"
+  assert_failure
+}