refactor: consolidate nginx.conf.sigil server blocks

The default nginx template rendered four near-identical server blocks per app, so the same listen, access_log, error_log, ssl_*, error_page, and proxy chain had to be maintained across http, https, grpc, and grpcs branches. Merging http and https into a single branch keyed on an `is_ssl` boolean, and likewise grpc and grpcs, removes the duplicate proxy_set_header chain and error-page locations and brings the structure in line with the openresty proxy template. Output is preserved up to whitespace and the existing `cat -s` pass already squashes the leftover blank lines. Adds plugins/nginx-vhosts/template_test.go exercising the rendered output via sigil as a Go library across HTTP-only, HTTPS, HTTP-to-HTTPS redirect, no-listeners 502 fallback, gRPC, gRPCs, gRPC-without-listeners skip, IPv4 bind, upstream blocks with and without keepalive, the access_log `off` short-circuit, the X-Forwarded-Ssl toggle, the http2 listen-parameter vs directive split, http2_push_preload conditional emission, and the nginx.conf.d/*.conf include in every code path; and tests/unit/nginx-vhosts_16.bats covering deploy plus `nginx -t` end-to-end for both HTTP and HTTPS apps.

tests/unit/nginx-vhosts_16.bats

@@ -0,0 +1,72 @@
+#!/usr/bin/env bats
+
+load test_helper
+
+setup_local_tls() {
+  TLS=$BATS_TMPDIR/tls
+  mkdir -p $TLS
+  tar xf $BATS_TEST_DIRNAME/server_ssl.tar -C $TLS
+  tar xf $BATS_TEST_DIRNAME/domain_ssl.tar -C $TLS
+  sudo chown -R dokku:dokku $TLS
+}
+
+teardown_local_tls() {
+  TLS=$BATS_TMPDIR/tls
+  rm -R $TLS
+}
+
+setup() {
+  global_setup
+  [[ -f "$DOKKU_ROOT/VHOST" ]] && cp -fp "$DOKKU_ROOT/VHOST" "$DOKKU_ROOT/VHOST.bak"
+  create_app
+}
+
+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) refactored template generates nginx -t parseable http config" {
+  run deploy_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 "sudo nginx -t"
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+}
+
+@test "(nginx-vhosts) refactored template generates nginx -t parseable https config" {
+  setup_local_tls
+  run deploy_app
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+
+  run /bin/bash -c "dokku certs:add $TEST_APP $BATS_TMPDIR/tls/server.crt $BATS_TMPDIR/tls/server.key"
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+
+  run /bin/bash -c "sudo nginx -t"
+  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 "ssl_certificate"
+  assert_output_contains "return 301 https"
+  teardown_local_tls
+}
+