test(wpmc S4): restore ACF-missing guard assertions + tighten stub signature

Re-add ->never() to both noop tests so Brain Monkey/Mockery actively asserts
acf_add_local_field_group is never called when ACF is absent.  Order matters:
expect() must precede when('function_exists')->alias() so FunctionStub sees the
real function_exists and skips the eval() path (function already declared in
acf-stubs.php).  Also tighten stub signature to match real ACF ($field_group,
no type hint, no default).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/FieldGroupTest.php

@@ -19,17 +19,20 @@ final class FieldGroupTest extends TestCase
 
     public function test_do_register_is_noop_when_acf_missing(): void
     {
+        // Set up the ->never() expectation BEFORE aliasing function_exists, so that
+        // FunctionStub::__construct() sees the real function_exists and skips the
+        // eval() path (the function is already declared in acf-stubs.php).
+        \Brain\Monkey\Functions\expect('acf_add_local_field_group')->never();
         \Brain\Monkey\Functions\when('function_exists')->alias(function ($name) {
             return $name !== 'acf_add_local_field_group';
         });
 
-        // acf_add_local_field_group is NOT stubbed here: if do_register() called it,
-        // the Patchwork-instrumented stub from acf-stubs.php would run (a silent no-op),
-        // and we would detect the bug via the assertion below being vacuously unreachable.
-        // The ->alias() guard above is the behavioural assertion: do_register() must
-        // return before reaching the acf_add_local_field_group call.
         FieldGroup::do_register();
 
+        // The real check is the ->never() Mockery expectation above (verified at
+        // teardown). assertTrue is here only to keep PHPUnit from flagging the
+        // test as risky under failOnRisky="true" — Mockery assertions don't
+        // count toward PHPUnit's assertion tally.
         self::assertTrue(true);
     }